Constraining Geogenic Sources of Boron Impacting Groundwater and Wells in the Newark Basin, USA

Abstract

The Newark Basin comprises Late Triassic and Early Jurassic fluvio-lacustrine rocks (Stockton, Lockatong, Passaic, Feltville, Towaco, and Boonton Formations) and Early Jurassic diabase intrusions and basalt lava flows. Boron concentrations in private well water samples range up to 18,000 μg/L, exceeding the U.S. Environmental Protection Agency Health Advisory of 2000 μg/L for children and 5000 μg/L for adults. Boron was analyzed in minerals, rocks, and water samples using FUS-ICPMS, LA-ICP-MS, and MC ICP-MS, respectively. Boron concentrations reach up to 121 ppm in sandstone of the Passaic Formation, 42 ppm in black shale of the Lockatong Formation, 31.2 ppm in sandstone of the Stockton Formation, and 36 ppm in diabase. The δ¹¹B isotopic values of groundwater range from 16.7 to 32.7‰, which fall within those of the diabase intrusion (25 to 31‰). Geostatistical analysis using Principal Component Analysis (PCA) reveals that boron is associated with clay minerals in black shales and with Na-bearing minerals (possibly feldspar and evaporite minerals) in sandstones. The PCA also shows that boron is not associated with any major phases in diabase intrusion, and is likely remobilized from the surrounding rocks by the intrusion-related late hydrothermal fluids and subsequently incorporated into diabase. Calcite veins found within the Triassic rock formations exhibit relatively elevated concentrations ranging from 6.3 to 97.3 ppm and may contain micro-inclusions rich in boron. Based on the available data, it is suggested that the primary sources of boron contaminating groundwater in the area are clay minerals in black shales, Na-bearing minerals in sandstone, diabase intrusion-related hydrothermal fluids, and a contribution from calcite veins.

1. Introduction

Boron, a metalloid element, is widely distributed within minerals across the earth’s crust, with an average concentration of 8 parts per million (ppm). Boron in groundwater can originate from both natural geogenic sources and anthropogenic activities. According to guidelines set by the World Health Organization [1], the ideal boron concentration in drinking water should not exceed 500 μg/L. Furthermore, the World Health Organization has set a guideline for boron in drinking water at 2400 μg/L and established a daily intake limit for boron for adults at 0.17 mg/kg body weight [1]. A long-term threshold value of 500 μg/L has been suggested for boron concentration in irrigation water [2] and reference therein. In the Newark Basin area, boron in groundwater has concentrations exceeding the U.S. Environmental Protection Agency Health Advisory (2000 μg/L for children and 5000 μg/L for adults) [3,4] and reaching up to 24,000 μg/L in some areas [5]. Despite limited testing for boron in well water, Newark Basin studies revealed that 18 out of 376 well water samples (5%) contained high concentrations of this element [5,6].

Boron can pose serious health hazards when consumed in high concentrations. It can affect the male reproductive system, reducing testosterone levels in men, and may affect fetal development [7]. The presence of boron in groundwater may also pose a risk to plants when irrigated with concentrations over 1000 μg/L ([2] and references therein). In the Newark Basin, some sensitive species of plants irrigated with well water containing boron levels exceeding 1000 μg/L sustained damage [5].

In the rural and suburban communities of the western portion of the Newark Basin, including areas in and around Hunterdon, Mercer, and Somerset Counties in New Jersey, as well as Bucks and Montgomery Counties in Pennsylvania, many communities are entirely reliant on individual private wells and are at risk for boron contamination of well water. Because well owners are responsible for testing their own water and boron is an unregulated contaminant, most well owners are unaware of the risks of boron exposure and the need to test their water.

Boron is difficult to remove from water, as testing has shown that reverse osmosis only removes about 15% of the boron in well water. The New Jersey Geological and Water Survey (NJGWS) tested and recommended a combination of reverse osmosis followed by mixed-bed deionization as an effective boron removal system for point-of-use applications, specifically for drinking and cooking water. While whole-house water treatment would be ideal, as boron is likely absorbed through the skin, a system utilizing reverse osmosis followed by mixed-bed deionization media is expensive and impractical for whole-house water treatment.

A better option is to investigate the well using geophysical methods and test the various water-bearing zones to identify the zone contributing the boron. Once identified, sealing off that zone from the well would solve the problem at its source and eliminate boron exposure throughout the home [5,6]. Therefore, determining the specific rocks and minerals responsible for boron contamination in groundwater is essential for developing targeted remediation strategies and protecting public health.

Previous researchers suggested evaporites and datolite hosted in diabase, as well as Early Jurassic magmatic activity as possible sources of boron [8]. We believe that sandstone and black shales of the Triassic Formations, as well as numerous Early Jurassic diabase intrusions and the associated sulfides (pyrite and chalcopyrite) and calcite, could be possible sources of boron. The numerous calcite veins hosted in the Triassic rocks are also a possible source of boron. The purpose of this project is to identify the potential source(s) of boron that contaminate groundwater and wells in the Newark Basin of New Jersey and Pennsylvania. Trace and major elements were analyzed in the investigated rocks (sandstone, black shales, diabase), and statistical analysis via Principal Component Analysis (PCA) was adopted to identify the inorganic and/or organic phases with which boron is associated. Boron and its isotopic compositions were also analyzed in groundwater.

2. Geology and Regional Hydrogeology of the Newark Basin

The Newark Basin, situated in the northeastern United States, stands as one of the most extensively studied rift basins within the Newark supergroup [9,10] (Figure 1). Stretching over 190 km in length and up to 50 km in width, it forms an elongated half-graben (Figure 2A) [10,11,12]. To its northeast and northwest, the basin is bordered by Precambrian and Early Paleozoic rocks, a continuation of the southwestern Appalachian segment of the New England upland. Meanwhile, to its southeast lie the Paleozoic and Precambrian Appalachian highlands of the Blue Ridge and Piedmont provinces [13]. These Appalachian highlands resulted from the Paleozoic collisions between the North American continent and various Gondwanan fragments, including Africa, culminating in the formation of the supercontinent Pangea [14].

The Newark Basin experienced rifting during the Pangea breakup and the initiation of the Atlantic Ocean opening [15]. This continental rifting prompted Triassic extensional tectonic activity, leading to the reactivation of Paleozoic NE-SW-trending major faults and the subsequent formation of a half-graben [16,17]. Sitting atop the Precambrian–Paleozoic basement are Triassic–Jurassic fluvio-lacustrine formations (6000 m to 8000 m) with occasional diabase intrusions and basaltic flows (Figure 2B) [10,11], listed and briefly described below.

Stockton Formation (1800 m): Comprising alluvial/fluvial arkosic and sandstone facies.

Lockatong Formation (1000–2000 m): Dominated by cyclic organic matter-rich black shale and grey to red mudstone, siltstone, and sandstone facies with intercalations of argillaceous carbonate.

Passaic Formation (2800 m): Predominantly consisting of red mudstone and sandstone containing evaporites with minor lacustrine gray and black shale beds.

Jurassic rocks (300–400 m): Comprising clastic sedimentary rocks intercalated with basaltic flows and intersected by diabase intrusions.

Research conducted on the Triassic-Jurassic fluvio-lacustrine rocks by Herman [18] and investigations on cores from the Newark Basin Coring Project (NBCP) by Olsen et al. [10] have unveiled the presence of numerous calcite veins within Early Mesozoic fluvio-lacustrine formations of Stockton, Lockatong, and Passaic. These calcite veins were formed during the Triassic–Jurassic time [18] as a result of fluid mixing between heated meteoric water and cooler formational waters [12].

Besides the geological context presented above, the hydrogeological framework is important when assessing the circulation of groundwater [19]. The hydrogeological context of the Newark Basin is characterized by fractured bedrock aquifers, which include the Stockton, Lockatong, and Brunswick aquifers [20]. The Brunswick aquifer encompasses any formation younger than the Lockatong Formation in the Newark Basin. Geophysical investigations, including borehole imaging, have identified and classified water-bearing features (WBFs) within these aquifers into three main categories: bedding planes and layers, fracture planes, and linear intersections of bedding and fracture planes [20].

Northern areas of the Newark Basin have been glaciated, and in these areas, the overburden may be composed of glacial till, stratified drift, or other glacial deposits. Outside the glaciated areas, the saprolite can be relatively thick. The water table is typically 3 to 6 m below land surface in overburden or shallow, highly weathered bedrock. Recharge occurs from precipitation, especially where WBFs intersect the land surface. Groundwater flow dynamics differ between shallow and deep bedrock (generally greater than 5.5 m below ground surface), with the former exhibiting uniformly distributed flow aligned with the topographic slope and the latter displaying anisotropic, confined, or semi-confined flow controlled by gently dipping strata ([20] and references therein).

Stratigraphic bedding exerts dominant control on groundwater storage and movement throughout the basin, with vertical circulation (leakage) between water-bearing zones also influenced by joints ([20] and references therein). Transmissivity varies significantly among Newark Basin aquifers and within discrete sections of each aquifer ([20] and references therein). Public supply and private wells are usually in the 61 to 183 m-deep range and intersect a variety of WBFs. Geophysical logging and inflatable packer testing have shown that certain WBFs contribute significantly more boron than others, and that grouting off these zones has greatly reduced boron concentrations in several wells [6].

3. Materials and Methods

The samples for this study were collected from the Stockton, Lockatong, and Passaic Formations, a diabase quarry, and calcite veins. The samples were extracted from outcrops as well as the Nursery and Titusville cores of the Newark Basin Continental Drilling Project (NBCDP) encompassing a total of 6770 m of core material.

From the diabase quarry, located in Moores Station (Figure 3) (Latitude: 40°19′29.96″ N, Longitude: 74°54′37.71″ W), diabase samples (n = 4) were collected along with the associated chalcopyrite (n = 5), pyrite (n = 5) and calcite (n = 7). Samples of Sandstone (n = 2) of the Stockton Fm., black shale (n = 3) and Limestone (n = 1) of Lockatong Fm., and sandstone (n = 2) of the Passaic Fm. were also collected (

Table 1. The coordinates of the analyzed samples for Triassic rocks (Stockton, Lockatong, and Passaic formations) and the calcite veins, as well as the depth at which they were collected.

Sample	Formation	Mineral/Rock	Core/Outcrop	Coordinates	Depth (m)
PN33a	Passaic	Calcite/Sandstone
PN33b		Calcite/Sandstone	Outcrop	40.338500° N, 74.792700° W	0
PN33c		Calcite/Sandstone
N430.3	Lockatong	Black shale	Nursery	40.300861° N, 74.824230° W	131.2
N362.4b		Limestone	Nursery		110.5
T2671b		Black shale	Titusville	40.326379° N, 74.850545° W	814.1
T2671L		Limestone	Titusville
PN104a		Calcite	Outcrop
PN104b		Calcite		40.338500° N, 74.792700° W	0
PN104c		Calcite
CaMDn1, 1a	Stockton	Calcite/Sandstone	Racer	40.265096° N, 74.805383° W	61
CaMDn2, 2a		Calcite/Sandstone	Racer	40.265096° N, 74.805383° W	73.8

). Calcite that occurs in numerous veins was collected from the Stockton Fm. (n = 5), the Lockatong Fm. (n = 3), and the Passaic (n = 3) (Table 1).

Samples of diabase (n = 4), pyrite (n = 7), chalcopyrite (n = 7), and calcite (n = 7) were carefully crushed and handpicked using a binocular microscope and microdrill. Trace element analysis in diabase were performed at Actlabs using PGNAA (Prompt Gamma Neutron Activation Analysis). Boron was analyzed at Actlabs using the Fusion-Inductively Coupled Plasma Mass Spectrometry (FUS-ICPMS) method. The reader is referred to www.Actlabs.com (accessed on 28 April 2020) for a detailed description of these methods.

To determine whether pyrite, chalcopyrite, and calcite associated with diabase contain boron-bearing inclusions, grains of these minerals underwent analysis for both trace and major elements using laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) at the Memorial University facility in Newfoundland, Canada. Images of specific grains were utilized to direct the LA-ICP-MS analyses. An ArF Excimer LA system operating at a wavelength of 193 nm was paired with an Element XR ICP-MS. Spot analyses were conducted to target specific locations, with single spot analyses utilizing 5 pulses/s (5 Hz) and producing a crater size of 20 μm. A helium carrier gas (flow rate of 1 L/min) was utilized to transport the ablated material, which was mixed coaxially with Ar before entering the ICP torch. Concentration and detection limit calculations followed the methodology outlined by Longerich et al. [21]. Reference materials (MASS-1 and NIST SRM 610) were analyzed twice at the beginning and end of each analytical session, as well as after every 20 measurements on samples. Regarding calcite found in the veins, analyses were carried out at the Pacific Centre for Isotopic and Geochemical Research (PCIGR) in the University of British Columbia using a ArF excimer Laser (193 nm, ASI Resolution M50LR), connected to a Quadrupole ICP-MS (Agilent 7700x). Calcite samples were ablated using a 120 μm spot size and a repetition rate of 8 Hz. Energy density was set to 1.8 J/cm². Helium was used as carrier gas and N₂ was admixed for higher intensity. The instrument was tuned for maximum sensitivity, low oxide rates (ThO/Th < 0.3%) optimum mass bias (238/232 ~ 1.05), and high stability (RSD on 139 < 3%). Calcite samples ablated better but seemed to be inhomogeneous; during ablation, several phases seemed to be mobilized. During data reduction, the phases with higher Ca indicating carbonate phases were selected. Data reduction was carried out using the software program Iolite v. 2.5. with background subtraction, data integration and calibration. For the calcite samples internal as well as external calibration was performed using Ca = 38% m/m and the synthetic glass standard SRM NIST612. The synthetic glass standard SRM NIST610 and the USGS carbonate reference material MACS-3 were used as quality control.

Groundwater samples were collected for boron isotopic analysis from private wells in the Passaic Formation in the Hunterdon Plateau and adjacent to diabase in the Lambertville, New Jersey area. Two well water samples were analyzed for their B isotope compositions and boron concentrations using a Nu Plasma II multicollector inductively coupled plasma mass spectrometer (MC ICP-MS) at the Isotope Ratio Mass Spectrometry Laboratory, Department of Geosciences, Stony Brook University, Stony Brook, NY. Sample aliquots were processed through column chromatography to isolate only B from the sample following a procedure modified from Lemarchand et al. [22]. Accuracy is provided by certified SRM 951 values. The standard used to correct the sample for in-run plasma-induced mass fractionation is a 50 ppb solution of the NIST SRM 951 boric acid. In addition, background was monitored using the same nitric acid and deionized water that was used to dilute the samples and SRM 951 standards, and subtracted from all analysis. The SRM 951 boric acid standard has a certified ¹¹B/¹⁰B value of 4.0437 ± 0.003. Results were corrected for background, drift and fractionation, and final ¹¹B/¹⁰B ratio. Seawater was run alongside all unknowns through chemical preparation and analysis. Sample aliquots were spiked with NSIT SRM 952, thoroughly mixed and then processed through column chromatography to isolate only B. The sample was then analyzed on the MC ICP-MS and the B concentration was calculated based on the measured isotopic composition and known isotope dilution through standard calculations. The SRM 952 standard has a certified ¹¹B/¹⁰B value of 0.0532 ± 0.0005.

Principal Component Analysis (PCA) was conducted using SPSS 23, a statistical software package developed by IBM, based in Armonk, New York. PCA is widely utilized as an effective method for studies related to water quality [23,24]. The goal of PCA is to identify the specific mineral phases in different rock types (sandstone, shale, limestone, and diabase) to which boron is linked. Understanding these associations is crucial for determining the sources of boron that contaminates groundwater passing through these rock formations. In this analysis, two principal components (PC1 and PC2) were extracted, and Varimax normalized rotation was applied to derive the loading values for each parameter within these components. Each principal component encompasses a set of correlated parameters with both positive and negative loadings, facilitating the interpretation of the primary geochemical processes influencing the dataset. PCA effectively identifies clusters of related elements. The first two principal components together account for a significant portion of the variance, specifically 94.091% for shale-limestone, 80.004% for sandstone, and 93.759% for diabase. PC1 alone accounts for a substantial part of the variance in each case, highlighting its major role in explaining the dataset’s variability.

4. Results

4.1. Geochemical Data

The results of geochemical analysis conducted on the rocks of the Triassic formations, calcite veins, diabase, and the associated minerals are provided in

Table 2. Concentrations of major elements (%) for the Triassic rocks, Newark Basin (* Total Organic Carbon (TOC) values (in wt%) are from Rddad [9]).

Sample	Rock	Formation	SiO2	Al2O3	Fe2O3(T)	MnO	MgO	CaO	Na2O	K2O	TiO2	P2O5	LOI	TOC *
CaMDn1	Sandstone	Stockton	73	9.13	2.94	0.089	1.59	3.3	4.01	0.11	0.467	<0.01	4.13
CaMDn2	Sandstone	Stockton	77.45	7.7	1.08	0.03	0.61	2.09	3.57	0.31	0.316	0.02	4.11
N430.3	Black shale	Locktong	44.89	14.18	5.85	0.093	5.29	6.21	3.4	5.88	0.624	0.04	11.67	0.79
N362.4b	Black shale	Locktong	41.98	13.36	4.92	0.087	6.73	7.69	3.54	4.43	0.497	0.08	11.85	2.72
T2671b	Black shale	Locktong	48.09	15.79	5.48	0.062	5.12	4.21	4.13	5.34	0.619	0.1	11.99	1.90
T2671L	Limestone	Locktong	17.33	5.25	7.25	0.263	13.48	20.9	1.4	2.17	0.187	0.03	3.21	0.50
PN33a	Sandstone	Passaic	55.95	19.02	7.63	0.036	2.42	1.03	6.8	2.62	0.821	0.09	2.87
PN33b	Sandstone	Passaic	56.98	18.42	7.41	0.033	2.71	0.73	6.54	2.71	0.794	0.14	2.45

,

Table 3. Concentrations of trace elements, including boron, in ppm for the Triassic rocks, Newark Basin.

Sample	Rock	Formation	B	Sc	Be	V	Cr	Co	Ni	Cu	Zn	As	Sr	Zr	Nb	Mo	Ag	Sn	Sb	Ba	Tl	Pb	Th	U
CaMDn1	Sandstone	Stockton	24	9	1	36	20	10	<20	10	<30	<5	87	144	5.1	<2	<0.5	1	<0.2	67	0.1	7	6.3	1.5
CaMDn2	Sandstone	Stockton	31	2	<1	30	<20	3	30	<10	220	<5	75	86	4	<2	<0.5	<1	<0.5	569	<0.1	33	4.3	1
N430.3	Black shale	Lockatong	36	14	2	165	80	18	70	60	1470	30	608	73	10	3	<0.5	2	<0.2	3027	1	20	5	6.1
N362.4b	Black shale	Lockatong	42	19	3	173	70	15	50	70	110	8	659	66	9	30	<0.5	2	<0.5	254	0.3	16	7	11
T2671b	Black shale	Locktaong	32	12	3	162	80	16	40	60	90	7	322	82	10	>100	<0.5	2	0.9	372	0.7	12	9.1	13
T2671L	Limestone	Lockatong	16	24	2	116	30	7	30	30	120	<5	1679	25	3	70	<0.5	<1	<0.5	346	0.2	14	4.8	7.2
PN33a	Sandstone	Passaic	101	16	2	87	90	21	50	<10	110	9	97	146	13	<2	<0.5	3	0.8	892	0.6	25	14	1.8
PN33h	Sandstone	Passaic	121	17	2	94	90	24	60	20	140	8	75	144	14	<2	<0.5	4	0.9	344	<0.1	26	13	1.9

,

Table 4. Concentrations of major elements (%) for the Early Jurassic diabase rocks, Newark Basin.

Sample	SiO2	Al2O3	Fe2O3(T)	MnO	MgO	CaO	Na2O	K2O	TiO2	P2O5	LOI
DoP 1	43.26	14.19	13.52	0.11	10.21	3.22	1.39	2.56	1.338	0.11	10.68
DoP 2	38.59	12.71	13.07	0.128	10.85	7.25	1.68	1.11	1.08	0.10	12.16
DoP 3	41.98	14.14	12.99	0.108	11.1	3.68	1.49	2.44	1.222	0.11	10.4
DoP 4	42.5	14.54	12.88	0.108	11.02	3.75	1.53	2.6	1.205	0.12	10.11

and

Table 5. Concentrations of trace elements, including boron, in ppm, for the Early Jurassic diabase rocks, Newark Basin.

Sample	B	Sc	Be	V	Cr	Co	Ni	Cu	Zn	As	Rb	Sr	Zr	Nb	Mo	Ag	Sn	Sb	Ba	Pb	Th	U
DoP 1	25	42	<1	318	310	55	110	110	80	<5	79	103	112	7.8	<2	<0.5	1	<0.2	1307	10	2.45	0.72
DoP 2	23.9	34	<1	264	240	46	80	100	80	<5	30	90	91	6.2	<2	<0.5	1	<0.2	338	6	1.88	0.71
DoP 3	27.9	41	<1	273	280	50	90	120	80	<5	63	109	103	7.1	<2	<0.5	1	<0.2	1003	9	2.16	0.8
DoP 4	35.6	40	<1	273	270	49	90	110	90	<5	65	109	103	6.9	<2	<0.5	1	<0.2	1058	7	2.16	0.76

and the Supplementary Materials (Tables S1 and S2). Samples of Stockton and Passaic sandstone exhibit boron concentrations ranging from 23.9 to 31.2 ppm and 101 to 121 ppm, respectively. These values exceed the typical concentrations found in the Earth’s crust, which is around 8 ppm. Some trace elements show relatively high concentration of V (30–94 ppm), Zn (<30–220 ppm), Zr (86–146 ppm), Ba (67–392 ppm) in sandstone. High concentrations of major elements such as SiO₂ (55.95–77.45%), Al₂O₃ (7.7–19.02%), Fe₂O₃ (1.08–7.63%), and Na₂O (3.57–6.8%) are observed. Additionally, relatively high concentrations of trace element Sr (75 ppm to 97 ppm) are recorded. Black shale samples collected from the Lockatong Formation display varying boron contents ranging from 15.6 ppm to 42 ppm, as well as other trace elements such as V (116–173 ppm), Cr (30–80 ppm), Mo (3–>100 ppm), Zn (90–1470 ppm), Sr (322–169 ppm), and Ba (254–3027 ppm). These samples exhibit comparable concentrations of major elements (SiO₂ = 41.98–48.09%, Al₂O₃ = 13.36 –15.79%, Fe₂O₃ = 4.92–5.85%, MgO = 5.12–6.73%, CaO = 4.21–7.69%, Na₂O = 3.4–4.13%, and K₂O = 4.43–5.88%) and higher concentrations of Sr (322–1679 ppm) compared to sandstone samples. Calcite veins from the Stockton, Lockatong, and Passaic formations show boron concentrations in the range of 6.3 to 7.7 ppm, 7.8 to 97.3 ppm, and 6.6 to 7.5 ppm, respectively (Table S1). Abnormally high boron contents are recorded in one sample of Stockton calcite (6,980 ppm) and one Passaic calcite (12,100 ppm). Diabase samples exhibit concentrations of boron ranging from 23.9 ppm to 35.6 ppm.

The major elements display high concentrations (SiO₂ = 18.6–43.3%, Al₂O₃ = 12.7–14.5%, Fe₂O₃ = 12.5–13.5%, MgO = 10.2–11.1%), typical of a diabase composition. The trace elements that show relatively high concentrations are V (264–318 ppm), Cr (240–310), Ni (90–110 ppm), Cu (100–120 ppm), Co (46–55 ppm), Sr (90–103 ppm), and Ba (338–1307 ppm).

Sulfides associated with diabase display lower boron concentrations, reaching up to 7.4 ppm and 3.9 ppm in chalcopyrite and pyrite, respectively (Table S2). Arsenic, which also contaminates groundwater [9,25], exhibits high concentrations ranging from 982 to 9572 ppm in pyrite. Other trace elements exhibit very low concentrations. In calcite associated with the diabase intrusion, boron concentrations vary from 5 ppm to 14.6 ppm, while lithium content ranges from <3 ppm to 107 ppm (Table S2). High concentrations of titanium (up to 26.9%) and SiO₂ (up to 25.5%) suggest the presence of Ti- and Si-rich inclusions likely derived from the diabase.

4.2. Boron Isotope

Boron isotope compositions and boron contents are reported in

Table 6. Boron concentrations and boron isotope data in groundwater flowing in the Passaic Formation. The average of the seawater B isotope values (n = 31) is also given with the published value from Foster et al. [26].

Sample	Location	11B/10B	δ11B (‰)	2σ (ppm)	B (ppm)	2σ (‰)
AT18MS	Hunterdon Plateau	4.18	32.7	0.5	15.3	0.5
L247SF	Adjacent to diabase, Lambertville	4.11	16.7	0.7	7.67	0.03
Seawater average		4.20	39.6	0.5
Published value		4.20	39.61	0.04

. δ¹¹B is expressed as

$${\mathsf{\delta }}^{11}\mathrm{B}=({\displaystyle \frac{\frac{11\mathrm{B}}{10\mathrm{B}} sample}{\frac{11\mathrm{B}}{10\mathrm{B}} standard}}-1)\times 1000$$

The analyzed groundwater flowing through the Passaic formation displays δ¹¹B isotopic values ranging from 16.7 to 32.7‰. These isotopic compositions fall within those of groundwater samples collected in the Pennsylvania portion of the Newark Basin by USGS from 40 private supply wells (δ¹¹B = 1.2 to 23.8‰) and overlap those of the diabase intrusion (25 to 31‰) [8]. Boron contents in the analyzed water samples vary from 7.67 ppm (7670 μg/L) to 15.3 ppm (15,300 μg/L).

4.3. Geostatistical Analysis

The result of the geostatistical (PCA) analysis conducted on the rocks of the Late Triassic formations (sandstone, shale, limestone) and the diabase are reported in Figure 4, Figure 5 and Figure 6 and Table 7, Table 8 and Table 9.

4.3.1. PCA in Shale-Limestone

Principal Component Analysis (PCA) for the shale-limestone dataset reveals significant correlations among various elements (Figure 4, Table 7). PC1 captures 67.857% of the variance, while PC2 explains 26.234%, together accounting for a cumulative variance of 94.091%. In PC1, major elements such as SiO₂ (0.995), Al₂O₃ (0.993), Na₂O (0.990) show strong positive loadings. Boron also exhibits a notable positive loading (0.888) on PC1. Total organic carbon (TOC), iron oxides, and carbonates are clustered in different quadrants with different loading values compared to boron (Figure 4, Table 7).

Figure 4. PCs biplot with loading values for 25 major and trace elements in black shale and limestone, Newark Basin.

Figure 4. PCs biplot with loading values for 25 major and trace elements in black shale and limestone, Newark Basin.

Table 7. Principal components (PCs) with loading values for 25 major and trace elements in shale and limestone, Newark Basin.

Variable	PC1	PC2
SiO2	0.995	0.06
Al2O3	0.993	0.021
Fe2O3	−0.916	0.2
MgO	−0.987	−0.141
CaO	−0.994	−0.047
Na2O	0.990	−0.111
B	0.888	0.08
Sc	−0.874	−0.142
Be	0.634	−0.759
V	0.962	0.059
Cr	0.982	0.163
Co	0.949	0.315
Ni	0.602	0.769
Cu	0.941	−0.025
Zn	0.246	0.966
As	0.387	0.922
Sr	−0.991	0.036
Zr	0.986	0.035
Nb	0.989	0.141
Mo	−0.928	−0.218
Ba	0.253	0.96
Pb	0.181	0.899
Th	0.647	−0.708
U	0.525	−0.844
TOC	0.65	−0.6
Variance (%)	67.857	26.234
Cumul. (%)	67.875	94.091

Table 7. Principal components (PCs) with loading values for 25 major and trace elements in shale and limestone, Newark Basin.

Variable	PC1	PC2
SiO2	0.995	0.06
Al2O3	0.993	0.021
Fe2O3	−0.916	0.2
MgO	−0.987	−0.141
CaO	−0.994	−0.047
Na2O	0.990	−0.111
B	0.888	0.08
Sc	−0.874	−0.142
Be	0.634	−0.759
V	0.962	0.059
Cr	0.982	0.163
Co	0.949	0.315
Ni	0.602	0.769
Cu	0.941	−0.025
Zn	0.246	0.966
As	0.387	0.922
Sr	−0.991	0.036
Zr	0.986	0.035
Nb	0.989	0.141
Mo	−0.928	−0.218
Ba	0.253	0.96
Pb	0.181	0.899
Th	0.647	−0.708
U	0.525	−0.844
TOC	0.65	−0.6
Variance (%)	67.857	26.234
Cumul. (%)	67.875	94.091

4.3.2. PCA in Sandstone

The PCA results for the sandstone dataset show that PC1 and PC2 explain 41.652% and 38.351% of the variance, respectively, summing to a cumulative variance of 80.004%. PC1 is characterized by high loadings of Na₂O (0.898), with boron displaying a strong positive loading (0.916) (Figure 5, Table 8). PC2 highlights high loadings for elements such as MgO (0.919) and CaO (0.903), but boron shows a moderate negative loading (−0.268).

Figure 5. PCs biplot with loading values for 25 major and trace elements in sandstone, Newark Basin.

Figure 5. PCs biplot with loading values for 25 major and trace elements in sandstone, Newark Basin.

Table 8. Principal components (PCs) with loading values for 25 major and trace elements in sandstone, Newark Basin.

Variable	PC1	PC2
SiO2	−0.545	−0.835
Al2O3	0.957	0.26
Fe2O3	0.94	0.31
MgO	0.335	0.919
CaO	−0.306	0.903
Na2O	0.898	−0.437
B	0.916	−0.268
Sc	0.796	0.479
Be	0.612	0.688
V	0.424	0.893
Cr	0.868	0.475
Co	0.952	0.245
Ni	0.692	0.502
Cu	0.224	0.957
Zn	−0.031	0.568
As	0.204	0.683
Sr	−0.011	0.987
Zr	0.418	−0.732
Nb	0.971	0.221
Mo	−0.025	0.623
Sn	0.995	0.051
Ba	−0.039	0.57
Pb	0.061	−0.255
Th	0.959	−0.189
U	0.113	0.83
Variance (%)	41.652	38.351
Cumul. (%)	41.652	80.004

Table 8. Principal components (PCs) with loading values for 25 major and trace elements in sandstone, Newark Basin.

Variable	PC1	PC2
SiO2	−0.545	−0.835
Al2O3	0.957	0.26
Fe2O3	0.94	0.31
MgO	0.335	0.919
CaO	−0.306	0.903
Na2O	0.898	−0.437
B	0.916	−0.268
Sc	0.796	0.479
Be	0.612	0.688
V	0.424	0.893
Cr	0.868	0.475
Co	0.952	0.245
Ni	0.692	0.502
Cu	0.224	0.957
Zn	−0.031	0.568
As	0.204	0.683
Sr	−0.011	0.987
Zr	0.418	−0.732
Nb	0.971	0.221
Mo	−0.025	0.623
Sn	0.995	0.051
Ba	−0.039	0.57
Pb	0.061	−0.255
Th	0.959	−0.189
U	0.113	0.83
Variance (%)	41.652	38.351
Cumul. (%)	41.652	80.004

4.3.3. PCA in Diabase

For the diabase dataset, PC1 and PC2 account for 59.612% and 34.148% of the variance, respectively, with a cumulative variance of 93.759%. In PC1, significant positive loadings are observed for elements such as SiO₂ (0.774), Fe₂O₃ (0.840), and TiO₂ (0.956) (Figure 6, Table 9). Boron, however, shows a negative loading (-0.204) on PC1, and a strong positive loading (0.884) on PC2 (Figure 6, Table 9). Instead, boron forms a cluster with uranium and zinc, as indicated by uranium’s strong loading (0.816) and zinc’s moderate loading (0.691) on PC2.

Figure 6. PCs biplot with loading values for 25 major and trace elements in diabase, Newark Basin.

Figure 6. PCs biplot with loading values for 25 major and trace elements in diabase, Newark Basin.

Table 9. Principal components (PCs) with loading values for 30 major and trace elements in diabase, Newark Basin.

Variable	PC1	PC2
SiO2	0.774	0.626
Al2O3	0.522	0.845
Fe2O3	0.840	−0.541
MnO	−0.552	−0.83
MgO	−0.807	0.517
CaO	−0.723	−0.69
Na2O	−0.927	−0.366
K2O	0.635	0.771
TiO2	0.956	0.293
P2O5	0.229	0.898
B	−0.204	0.884
Sc	0.778	0.614
V	0.977	−0.146
Cr	0.965	0.254
Co	0.994	0.112
Ni	0.991	0.059
Cu	0.357	0.717
Zn	−0.255	0.691
Ga	0.63	0.774
Rb	0.848	0.528
Sr	0.356	0.926
Zr	0.925	0.376
Nb	0.965	0.258
Ba	0.831	0.553
Hf	0.965	0.254
Ta	0.978	0.089
Tl	0.73	0.671
Pb	0.913	0.172
Th	0.96	0.271
U	−0.094	0.816
Variance (%)	59.612	34.148
Cumul. (%)	59.612	93.759

Table 9. Principal components (PCs) with loading values for 30 major and trace elements in diabase, Newark Basin.

Variable	PC1	PC2
SiO2	0.774	0.626
Al2O3	0.522	0.845
Fe2O3	0.840	−0.541
MnO	−0.552	−0.83
MgO	−0.807	0.517
CaO	−0.723	−0.69
Na2O	−0.927	−0.366
K2O	0.635	0.771
TiO2	0.956	0.293
P2O5	0.229	0.898
B	−0.204	0.884
Sc	0.778	0.614
V	0.977	−0.146
Cr	0.965	0.254
Co	0.994	0.112
Ni	0.991	0.059
Cu	0.357	0.717
Zn	−0.255	0.691
Ga	0.63	0.774
Rb	0.848	0.528
Sr	0.356	0.926
Zr	0.925	0.376
Nb	0.965	0.258
Ba	0.831	0.553
Hf	0.965	0.254
Ta	0.978	0.089
Tl	0.73	0.671
Pb	0.913	0.172
Th	0.96	0.271
U	−0.094	0.816
Variance (%)	59.612	34.148
Cumul. (%)	59.612	93.759

5. Discussion

Boron concentrations in the analyzed water samples range from 7800 to 15,300 μg/L, which fall within the range of <500 to 18,000 μg/L observed in most water samples determined by Rockafellow-Baldoni et al. [5] and Spayd [6] (Figure 7).

The following paragraphs highlight and discuss the possible source(s) of boron that contaminate groundwater. The PCA and geochemical data from the investigated rocks (sandstone, shale, limestone, and diabase) reveal distinct associations of boron. In sandstone, boron exhibits a significant positive loading on PC1 (loading = 0.916) alongside Na₂O (loading = 0.898), suggesting co-occurrence with sodium-bearing minerals, which may include feldspar and evaporite minerals given that the Passaic formation contains evaporites (gypsum and anhydrite). The elevated lithium content observed in groundwater [6] further corroborate the contribution of B-rich evaporite minerals. These boron-mineral phase association along with the high concentrations of boron (23.9–121 ppm) and the major oxide phase Na₂O recorded in sandstone samples suggest the adsorption onto and or the incorporation of boron into Na-bearing minerals (Figure 5) as suggested by several authors [8,27,28,29,30,31]. High boron concentrations are also observed in non-marine sandstone aquifers [32].

In shale-limestone, boron displays strong positive loadings (0.888) with major elements like SiO₂ (loading = 0.995) and Al₂O₃ (loading = 0.993) on PC1. These correlations indicate potential chemical affinities of boron within aluminosilicate phases (clay minerals), reinforcing its integration into silicate structures. Conversely, boron is not associated with total organic carbon (TOC), iron oxide (Fe₂O₃), or carbonate phases, as evidenced by the different loadings of these phases compared to boron (Figure 4). Varying levels of boron, ranging from about 80 to 600 ppm, are also observed in unaltered red shales of the Passaic Formation ([8] and references therein). Relatively elevated contents of B, Cr, and Zr registered for black shales suggest fixation of these elements in the inorganic fraction (shale) during diagenesis (Figure 4) [33,34,35,36,37,38]. Mather and Porteous [32] argued that the abundance of clay minerals predominantly influences the presence of boron.

Organic matter-poor limestone intercalations show lower boron concentration (15.6 ppm) compared to black shales, indicating that limestone did not adsorb boron and most other trace elements except for Sr and Sc as shown in the PCA biplot (Figure 4). Based on the aforementioned discussion, it can be inferred that clay minerals present in the Triassic rock formations might have acted as sources of boron contaminating the groundwater.

In diabase, boron with high concentrations (23.9–35.6 ppm) shows a negative loading on PC1 (loading = −0.204) and a strong positive loading on PC2 (loading = 0.884). Boron is not associated with major phases such as aluminosilicates, iron oxides, or carbonates as evidenced by different loadings values of these phases (Figure 6, Table 9). Boron’s association with uranium (loading = 0.816) and zinc (loading = 0.691) on PC2 suggests secondary associations possibly linked to remobilization processes (metasomatism) rather than a direct primary association with major mineral phases like aluminosilicates or carbonates. This indicates boron’s remobilization from the surrounding rocks (e.g., sandstone) by diabase-related hydrothermal fluids and its subsequent incorporation into new mineral phases in the diabase. This assertion is corroborated by the high concentrations of Zn in sandstone reaching up to 220 ppm. The contribution of diabase as a source of boron is supported by boron isotopic compositions. The δ¹¹B isotopic values (16.7 to 32.7‰) of groundwater are within those of the diabase intrusion (δ¹¹B = 25 to 31‰, [8]), corroborating the contribution of diabase as a potential source of boron.

The remobilization of boron from the surrounding host rocks was also proposed [8]. This remobilization may have caused the accumulation of hydrothermal sulfides (chalcopyrite, pyrite) and calcite within or near the diabase. Hence, it is possible that boron-rich inclusions may be incorporated within these sulfides. However, given that boron is typically not found in sulfides, the low boron concentrations (<8 ppm) in these sulfides suggest the probable absence of boron-rich micro-inclusions. Calcite, associated with diabase, exhibits boron concentrations reaching up to 14.6 ppm and lithium content up to 107 ppm. High concentrations of titanium (up to 26.9%) and silicon (up to 25.5%) suggest the presence of Ti- and Si-rich inclusions likely originating from the diabase.

Calcite veins from the Stockton, Lockatong, and Passaic formations exhibit boron contents in the range of 6.3–97.3 ppm, with very high values recorded in one Stockton calcite (6980 ppm) and one Passaic calcite (12,100 ppm). These abnormally high boron concentrations suggest either an analytical artifact or the presence of boron-rich micro-inclusions. Hence, it is possible that these calcite veins may contribute to boron contamination of groundwater.

Based on this discussion, a model is proposed for the fixation of boron into different potential mineral phases (Figure 8). During the Late Triassic, boron was sequestered from the Late Triassic lacustrine water and subsequently fixed in various mineral phases such as clay minerals and Na-bearing minerals (Figure 8A). In the Early Jurassic, the emplacement of the diabase intrusion led to the remobilization of boron from the surrounding rocks and its incorporation into new mineral phases within the diabase (Figure 8B). Concurrently in the Early Jurassic, calcite veins were also formed within the Late Triassic rocks. As groundwater flows through the aquifers, particularly along available open spaces such as bedding planes and fractures (WBFs), it leaches boron from the mineral phases (Figure 8C). Consequently, groundwater becomes contaminated with boron. The presence of vertical joints affecting the Triassic aquifers may facilitate the vertical flow of contaminated groundwater (leakage).

The identification of potential boron sources has significant implications for policymaking and public health measures. Recognizing these sources is crucial for determining, from a geological perspective, where to focus outreach efforts to alert private well owners that they need to test their water for boron. Currently, water treatment measures are necessary to prevent exposure and the adverse health effects of boron. The NJGWS recommends a comprehensive point-of-use boron removal system incorporating reverse osmosis, followed by mixed bed deionization (or a boron-specific resin) with a circulation pump, and concluding with a neutralizer and granular activated carbon block filter before the tap. Although this approach effectively lowers boron concentrations below the U.S. EPA health advisory level, it is costly and does not provide whole-house treatment. Alternatively, physically isolating the boron-containing water-bearing zones in the well could be the most effective and protective method for reducing boron levels in the long term [5,6]. Geologists must consider the potential presence of vertical joints that could facilitate the vertical movement of contaminated groundwater to maximize boron reduction by this technique. The importance of the current study lies in identifying the sources of boron, which is crucial for determining the most appropriate zones bearing boron-rich water to be targeted and ensuring the long-term reduction in boron levels in the well water.

6. Concluding Remarks

The study investigates potential sources of boron contamination in groundwater and wells of the Newark Basin, focusing on Triassic rock formations of Stockton, Lockatong, and Passaic, Jurassic diabase intrusions, and associated hydrothermal minerals. Geochemical and geostatistical analyses reveal that boron is associated with clay minerals in shale and with Na-bearing minerals (e.g., feldspar, evaporites) in sandstone. These inorganic fractions (clay minerals, Na-rich minerals) are potential sources of boron. Moreover, this study shows that late hydrothermal fluids related to the emplacement of diabase intrusion remobilized boron from surrounding rocks, resulting in relatively high boron concentrations ranging from 23.9 to 35.6 ppm observed in diabase, which indicates that it is a potential source of boron. This finding is supported by the isotopic composition of boron in groundwater (δ¹¹B = 16.7 to 32.7‰), which aligns with that of diabase intrusions (δ¹¹B = 25 to 31‰). Calcite veins hosted in the Triassic rock formations show relatively high concentrations (6.3–97.3 ppm) with the possible presence of boron-rich micro-inclusions, suggesting that these calcite veins can be considered contributors to boron contamination. The leaching of boron from these potential sources resulted in an enrichment of boron in the groundwater. The flow of boron-rich groundwater is facilitated by bedding planes and fractures in the aquifers.

The number of samples used in this project is relatively limited, and further studies with a larger sample size are required to generalize and consolidate our conclusions. Although this study provides insights into potential sources of boron such as diabase intrusions and clay and Na-bearing minerals of Triassic formations and diabase, other potential sources of boron such as mineral evaporites hosted in the Passaic Formation need to be explored, and that is the focus of our next project. With the potential sources of boron identified, the focus should then be on increasing awareness of the need to test well water for boron and implementing affordable and effective boron removal techniques, such as point-of-entry and point-of-use removal systems, as well as methods to isolate or block boron-containing water-bearing zones. A thorough evaluation of these approaches is necessary to develop robust solutions that mitigate risks associated with elevated boron levels.