Cambrian-Ordovician Aquifer System Research Articles

Strontium and radium occurrence at the boundary of a confined aquifer system

Aquifers are susceptible to contamination by naturally occurring metals and radionuclides that limits the quantity of groundwater available for drinking. The Midwestern Cambrian-Ordovician aquifer system (MCOAS) is an important drinking water source in the North Central region of the U.S., but can contain elevated levels of geogenic radium (Ra) and strontium (Sr). Here, the occurrence of Ra and Sr is investigated with respect to natural groundwater evolution and water-rock interactions using a multi-isotope approach (δ18O, δ2H, δ34SSO4, 87Sr/86Sr, and [234U/238U]), in a portion of the MCOAS that straddles regionally unconfined and regionally confined conditions. Hierarchical cluster analysis reveals three distinct groups of groundwater geochemistry, evolving from a young (<10,000 years) HCO3−- dominant groundwater located upgradient to old (>10,000 years) SO42− and Cl−-dominant groundwater located downgradient. Strontium and Ra concentrations are highest downgradient, where extreme [234U/238U] disequilibrium indicates increased water-rock interactions due to local confinement or a stagnant groundwater zone. Geochemical conditions evolve as a consequence of water-rock interaction, resulting in Ra and Sr mobilization mechanisms including enhanced mineral dissolution and weathering, and decreased sorption efficiency due to competitive sorption and the absence or dissolution of iron and manganese (hydr)oxides with sorbed Ra and Sr. Relative to redox processes, ‘mixed’ redox samples have the highest median Sr and Ra concentrations, where young, oxic groundwater mixes with old, anoxic groundwater over long open boreholes. Isotopic results (δ18O and δ2H, 87Sr/86Sr) indicate these wells with long open boreholes are receiving more of the old, deep groundwater elevated in Ra and Sr, suggesting that well construction is an important consideration for Ra and Sr occurrence. The presented framework can be used to examine the occurrence of geogenic contaminants with respect to recharge history, water-rock interactions, and evolving geochemical conditions in other aquifer systems with a range of groundwater residence times and geochemistry.

Correction to Association of Radionuclide Isotopes with Aquifer Solids in the Midwestern Cambrian–Ordovician Aquifer System

ADVERTISEMENT RETURN TO ISSUEPREVAddition/CorrectionNEXTORIGINAL ARTICLEThis notice is a correctionCorrection to Association of Radionuclide Isotopes with Aquifer Solids in the Midwestern Cambrian–Ordovician Aquifer SystemMadeleine MathewsMadeleine MathewsMore by Madeleine Mathews, Sean R. ScottSean R. ScottMore by Sean R. Scott, Madeline B. GotkowitzMadeline B. GotkowitzMore by Madeline B. Gotkowitz, and Matthew Ginder-Vogel*Matthew Ginder-VogelMore by Matthew Ginder-Vogelhttps://orcid.org/0000-0001-9183-1931Cite this: ACS Earth Space Chem. 2022, 6, 5, 1411–1412Publication Date (Web):April 13, 2022Publication History Received5 April 2022Published online13 April 2022Published inissue 19 May 2022https://doi.org/10.1021/acsearthspacechem.2c00037Copyright © 2022 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views122Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (501 KB) Get e-Alerts Get e-Alerts

Isotopic analysis of radium geochemistry at discrete intervals in the Midwestern Cambrian-Ordovician aquifer system

Radium (Ra) is a geogenic radioactive contaminant that frequently occurs at elevated levels in the Midwestern Cambrian-Ordovician aquifer system (MCOAS). Geochemical indicators (e.g., redox conditions or total dissolved solids) can broadly characterize conditions associated with elevated Ra levels in groundwater, but do not consistently correlate to elevated Ra within specific stratigraphic horizons. A coupled geochemical and isotopic study of groundwater and aquifer solids for major and trace elements, Ra, and uranium (U) at discrete intervals in the MCOAS was used to elucidate processes that may be responsible for this disconnect, via analysis of groundwater as well as extracted and digested solid aquifer samples. We find that the potential for Ra mobilization varies by stratigraphic unit, as observed by whole-rock 226Ra/238U (dis)equilibrium. Overall, the examined aqueous geochemical characteristics (e.g., redox conditions, total dissolved solids) do not explain Ra concentrations within the system, suggesting that alternative factors, like solid-phase associations or the extent of alpha recoil damage, may be more important. A relation between aqueous 87Sr/86Sr and 226Ra suggests that minerals with radiogenic 87Sr/86Sr are more likely to release 226Ra to the aqueous system. Overall, the release of U and Ra due to water-rock interaction varies with discrete stratigraphy, depending on aqueous geochemistry and available mineral associations. Due to complex Ra-rock interactions and the heterogeneous geology of the MCOAS, aqueous geochemistry does not fully predict the mobilization and concentration of Ra in groundwater. As sources and sinks of Ra within the MCOAS vary across stratigraphy, knowledge of aqueous geochemistry, available solid-phase associations, and nuclide leachability all are important to consider for understanding elevated Ra occurrence in aquifer systems.

Uncertain Waters: Participatory groundwater modelling in Chicago’s suburbs

Groundwater exists in underground aquifers and is largely hidden and intangible to water users. As such, groundwater models are one of the main vehicles through which groundwater is made legible. They are critical for water supply planning purposes. However, models are imperfect representations of limited data and contain much uncertainty, posing challenges for the water supply planning process. In this paper, we draw on a case from the Greater Chicago area to examine efforts by the authors and the Illinois State Water Survey to engage with local water managers to develop future water supply scenarios. Much of this area has been dependent upon the Cambrian-Ordovician Aquifer System for over 150 years. Over this period, water levels have declined by over 300 m and aquifers are expected to be unviable by 2030. Here we advance the growing field of participatory groundwater modelling (PGM) to identify forms of uncertainty and their influence on understandings of water supply and risk perceptions of depletion. Conceptually, we draw on the idea of models as world builders, where uncertainties are elucidated through knowledge production in the act of model building, while model development is simultaneously influenced by expectations, beliefs, and ambiguity surrounding those using the models. Through planning meetings and focus group discussions between groundwater modelers and water supply stakeholders, we identify four forms of interconnected uncertainty that hinder planning efforts: 1) hydrogeologic uncertainty, 2) modelling uncertainty; 3) water demand uncertainty; and 4) urban planning uncertainty. We describe our PGM efforts to reduce uncertainty and find stakeholder perceptions are as important as model uncertainties in water management decisions. Participatory modelling is effective in reducing and clarifying these four forms of uncertainty, particularly applied to short-term management decisions in a rapidly changing system. We conclude that future participatory modelling efforts need to focus on reducing communication barriers between scientists and local users.

Association of Radionuclide Isotopes with Aquifer Solids in the Midwestern Cambrian–Ordovician Aquifer System

Groundwater, an important source of drinking water globally, is susceptible to contamination by naturally occurring metals and radionuclides. Regional trends in groundwater quality are useful in pr...

Spatial and temporal variability of radium in the Wisconsin Cambrian–Ordovician aquifer system

AbstractCompliance monitoring of contaminants in public drinking water supplies results in publicly available data sets that are maintained by state agencies and useful for investigating changes in water quality. In this study, spatial and temporal trends in naturally occurring radium (combined 226Ra + 228Ra) concentrations in groundwater from the Midwestern Cambrian–Ordovician aquifer system were evaluated with a Wisconsin Department of Natural Resources compliance data set. Average combined Ra from 2000 to 2018 in the Midwestern Cambrian–Ordovician aquifer system increased from 5.5 to 7.9 pCi/L in the confined region and from 4.8 to 6.6 pCi/L in the unconfined region. On a local scale, the data demonstrate increasing radium in the communities of Sussex (n = 98) and Brookfield (n = 35), and a decreasing trend in nearby Waukesha (n = 176). This study demonstrates the utility of compliance data sets for investigating water quality trends and the importance of geographic scale in examining such data.

Effect of geochemical conditions on radium mobility in discrete intervals within the Midwestern Cambrian-Ordovician aquifer system

Radium (Ra) commonly occurs in groundwater obtained from the Midwestern Cambrian-Ordovician aquifer system (C-O-AS) at activities approaching and exceeding the United States Environmental Protection Agency's Maximum Contaminant Level (MCL) of 5 pCi/L for combined 226Ra and 228Ra. The occurrence of Ra(II) in groundwater is dependent on a number of factors, including the prevalence of parent radionuclides within hydrogeological strata, as well as aquifer geochemical conditions. Interbedded aquifer and aquitard sequences within the Midwestern C-O-AS are stratified with respect to Ra(II) activity and geochemical conditions, and thus the formations that serve as Ra(II) sources to groundwater remain poorly constrained. This study analyzes aqueous samples collected from short-screened wells at various depths within the Midwestern C-O-AS near Madison, WI, USA, to determine geochemical parameters including dissolved oxygen; pH; major and minor ions; and metals, including 226Ra, 228Ra, and parent isotopes 238U and 232Th. Additionally, the elemental composition of aquifer solids is determined as a function of depth. Within solid phases, 238U and 232Th occur in both fine-grained facies and as coatings on sandstone minerals. Most groundwater samples contain dissolved combined Ra(II) lower than 2.5 pCi/L; however, one well completed in the unconfined and one well completed in the confined portion of the groundwater system exceed 3.5 pCi/L. In the confined system, anoxic conditions are associated with elevated Ra(II) concentrations, while in the upper, oxic aquifer, elevated total dissolved solids are positively related to Ra(II). These results demonstrate that multiple factors impact Ra(II) sources and mobility in the regionally unconfined portion of the Midwestern C-O-AS.

A multidisciplinary-based conceptual model of a fractured sedimentary bedrock aquitard: improved prediction of aquitard integrity

A hydrogeologic conceptual model that improves understanding of variability in aquitard integrity is presented for a fractured sedimentary bedrock unit in the Cambrian-Ordovician aquifer system of midcontinent North America. The model is derived from multiple studies on the siliciclastic St. Lawrence Formation and adjacent strata across a range of scales and geologic conditions. These studies employed multidisciplinary techniques including borehole flowmeter logging, high-resolution depth-discrete multilevel well monitoring, fracture stratigraphy, fluorescent dye tracing, and three-dimensional (3D) distribution of anthropogenic tracers regionally. The paper documents a bulk aquitard that is highly anisotropic because of poor connectivity of vertical fractures across matrix with low permeability, but with ubiquitous bed parallel partings. The partings provide high bulk horizontal hydraulic conductivity, analogous to aquifers in the system, while multiple preferential termination horizons of vertical fractures serve as discrete low vertical hydraulic conductivity intervals inhibiting vertical flow. The aquitard has substantial variability in its ability to protect underlying groundwater from contamination. Across widespread areas where the aquitard is deeply buried by younger bedrock, preferential termination horizons provide for high aquitard integrity (i.e. protection). Protection is diminished close to incised valleys where stress release and weathering has enhanced secondary pore development, including better connection of fractures across these horizons. These conditions, along with higher hydraulic head gradients in the same areas and more complex 3D flow where the aquitard is variably incised, allow for more substantial transport to deeper aquifers. The conceptual model likely applies to other fractured sedimentary bedrock aquitards within and outside of this region.

Radium mobility and the age of groundwater in public-drinking-water supplies from the Cambrian-Ordovician aquifer system, north-central USA

High radium (Ra) concentrations in potable portions of the Cambrian-Ordovician (C-O) aquifer system were investigated using water-quality data and environmental tracers (3H, 3Hetrit, SF6, 14C and 4Herad) of groundwater age from 80 public-supply wells (PSWs). Groundwater ages were estimated by calibration of tracers to lumped parameter models and ranged from modern (<50 yr) in upgradient, regionally unconfined areas to ancient (>1 Myr) in the most downgradient, confined portions of the potable system. More than 80 and 40 percent of mean groundwater ages were older than 1000 and 50,000 yr, respectively. Anoxic, Fe-reducing conditions and increased mineralization develop with time in the aquifer system and mobilize Ra into solution resulting in the frequent occurrence of combined Ra (Rac = 226Ra + 228Ra) at concentrations exceeding the USEPA MCL of 185 mBq/L (5 pCi/L). The distribution of the three Ra isotopes comprising total Ra (Rat = 224Ra + 226Ra + 228Ra) differed across the aquifer system. The concentrations of 224Ra and 228Ra were strongly correlated and comprised a larger proportion of the Rat concentration in samples from the regionally unconfined area, where arkosic sandstones provide an enhanced source for progeny from the 232Th decay series. 226Ra comprised a larger proportion of the Rat concentration in samples from downgradient confined regions. Concentrations of Rat were significantly greater in samples from the regionally confined area of the aquifer system because of the increase in 226Ra concentrations there as compared to the regionally unconfined area. 226Ra distribution coefficients decreased substantially with anoxic conditions and increasing ionic strength of groundwater (mineralization), indicating that Ra is mobilized to solution from solid phases of the aquifer as adsorption capacity is diminished. The amount of 226Ra released from solid phases by alpha-recoil mechanisms and retained in solution increases relative to the amount of Ra sequestered by adsorption processes or co-precipitation with barite as adsorption capacity and the concentration of Ba decreases. Although 226Ra occurred at concentrations greater than 224Ra or 228Ra, the ingestion exposure risk was greater for 228Ra owing to its greater toxicity. In addition, 224Ra added substantial alpha-particle radioactivity to potable samples from the C-O aquifer system. Thus, monitoring for Ra isotopes and gross-alpha-activity (GAA) is important in upgradient, regionally unconfined areas as downgradient, and GAA measurements made within 72 h of sample collection would best capture alpha-particle radiation from the short-lived 224Ra.

Combining high resolution vertical gradients and sequence stratigraphy to delineate hydrogeologic units for a contaminated sedimentary rock aquifer system

Summary Hydrogeologic units (HGUs), representing subsurface contrasts in hydraulic conductivity, form the basis for all conceptual and numerical models of groundwater flow. However, conventionally, delineation of these units relies heavily on data sets indirect with respect to hydraulic properties. Here, we use the spatial and temporal characteristics of the vertical component of hydraulic gradient (i.e., vertical gradient) as the primary line of evidence for delineating HGUs for Cambrian–Ordovician sedimentary rocks at a site in Dane County, Wisconsin. The site includes a 16 km2 area encompassing a 3 km long mixed organic contaminants plume. The vertical gradients are derived from hydraulic head profiles obtained using high resolution Westbay multilevel systems installed at 7 locations along two, orthogonal 4 km long cross-sections and monitoring to depths between 90 and 146 m with an average of 3–4 monitoring zones per 10 m. These vertical gradient cross-sections reveal 11 laterally extensive HGUs with contrasting vertical hydraulic conductivity (Kv). The position and thickness of the Kv contrasts are consistently associated with sequence stratigraphic features (maximum flooding intervals and sequence boundaries) distinguished at the site using cores and borehole geophysical logs. The same sequence stratigraphic features are also traceable across much of the Cambrian–Ordovician aquifer system of the Midwest US. The vertical gradients and sequence stratigraphy were arrived at independently and when combined provide a hydraulically calibrated sequence stratigraphic framework for the site. This framework provides increased confidence in the precise delineation and description of the nature of HGU contacts in each borehole, reduced uncertainty in interpolation of the HGUs between boreholes, and some capability to predict HGU boundaries and thickness in offsite areas where high resolution hydraulic data sets are not available. Consequently, this HGU conceptual model will serve as a better basis for numerical simulations of groundwater flow and contaminant transport needed for continued risk assessment and to evaluate potential remedial technologies for the site. More broadly, the association between Kv contrasts and sequence stratigraphy demonstrated at this site provides valuable insight into the relationship between geology and hydraulic contrasts that can be transferred to other areas of the upper Midwest US and perhaps elsewhere to strata deposited in similar settings.

Implications of carbonate-like geochemical signatures in a sandstone aquifer: Radium and strontium isotopes in the Cambrian Jordan aquifer (Minnesota, USA)

Water–rock interaction in sandstone aquifers can potentially mobilize U-and Th-series radionuclides, resulting in elevated levels of radioactive constituents such as radium (Ra) in groundwater. Previous studies have delineated the sources of Ra in groundwater by assuming that 228Ra/226Ra of groundwater parallels the 232Th/238U activity ratio of aquifer solids. We present an integrated geochemical and isotopic study of groundwater and aquifer solids for major and trace elements, radium (224Ra, 226Ra, 228Ra), strontium isotopes (87Sr/86Sr), and tritium in the Cambrian Jordan quartz sandstone aquifer of southeastern Minnesota (USA). The Jordan aquifer, part of the Cambrian–Ordovician aquifer system of the upper midwestern USA, is characterized by a wide range of Ra activities in groundwater. Uniform Ca–Mg–HCO3− major element composition and patterns in 87Sr/86Sr indicate that interaction with carbonate minerals controls water chemistry. At least one contributing Sr source is radiogenic, yielding 87Sr/86Sr ratios up to 0.71347. Radium-226 activities exhibit especially large variations (<1–418mBqL−1), and the highest radium activities occupy a 100-km-long north–south region. Consistent with the carbonate-like elemental and Sr isotope signatures, 226Ra activity (median 85mBqL−1) is higher than 228Ra (median 47mBqL−1), yielding low 228Ra/226Ra ratios (median 0.5) that would be expected from a lithology preferentially containing U relative to Th, such as carbonate rocks. Collectively, results indicate the primary source of radium to the aquifer is alpha recoil from aquifer solids rather than external inputs of high-Ra waters, indicated by (1) similar 228Ra/226Ra ratios in aquifer solids and groundwater, including low, carbonate-like values in the high-226Ra samples; (2) some association between whole-rock 226Ra levels and groundwater 226Ra activities; and (3) groundwater 224Ra/228Ra averaging near unity. Furthermore, hydrogeologic setting (e.g. presence or absence of overlying carbonate units) is inconsistent with systematic inputs of high-Ra waters from overlying carbonate aquifers. The most proximate source of carbonate material in the sandstone aquifer seems to be the calcite and dolomite cement that yields different geochemical and isotopic fingerprints than silicate minerals would impart. Whole-rock 226Ra/238U≈1 implies that any U enrichment in the carbonate cement occurred at least 350,000years ago. High groundwater Ra activities relative to solids are attributable to anoxic conditions that play a role in maintaining inefficient radium removal for a fresh groundwater system, indicated by the estimated dimensionless whole-rock Ra distribution coefficient of 150–920 (median 560). Coprecipitation of Ra into barite is a potential but unquantified Ra removal mechanism. Overall, radium in the Jordan aquifer provides an example of a naturally-occurring trace element of health significance that may be derived not mainly from the quartz sandstone matrix, but from carbonate material such as calcite and dolomite cement. Especially when combined with complementary tracers such as 87Sr/86Sr, this study demonstrates the usefulness of the 228Ra/226Ra ratio as an indicator of the key mineral phases (silicate vs. carbonate) involved in supplying radium to groundwater.