Jean E. Moran

Origin and history of waters associated with coalbed methane: 129I, 36Cl, and stable isotope results from the Fruitland Formation, CO and NM

Abstract The Fruitland Formation of the San Juan Basin was deposited during the late Cretaceous and is associated with significant reservoirs of coalbed methane (CBM). The purpose of this study is to determine the origin and history of waters associated with the formation, using long-lived cosmogenic and stable isotope systems. Ratios of 129 I/I and stable isotope values (δD and δ 18 O) were determined in waters from close to 100 wells, 36 Cl/Cl ratios for a subset of these samples. A significant group of samples has 129 I/I ratios between 100 × 10 −15 and 200 × 10 −15 , indicating minimum iodine ages close to 60 Ma. If these ages are corrected for the addition of fissiogenic 129 I, they are compatible with the depositional age of the Fruitland Formation (Late Cretaceous). Several sets of waters are clearly present within the data. A group dominated by infiltration of recent surface waters is restricted to the uplifted basin margins, with a lateral extent of less than 5 km from outcrop, and is characterized by 129 I/I ratios in excess of 1500 × 10 −15 and meteoric δD, δ 18 O, and 36 Cl/Cl signatures. The rest of the basin is characterized by several subsets of formation waters which have undergone variable degrees of iodine enrichment through diagenesis as well as variable degrees of dilution. The first subgroup is found in coals of relatively low vitrinite reflectance and moderate enrichment of iodine. This subgroup predominantly consists of entrapped pore fluids, although it may also contain waters which infiltrated the coals at the time of the Laramide uplift, between 25 and 30 Ma. A second subgroup consists of formation waters associated with coals of high vitrinite reflectance. Despite subsequent uplift, the high iodine concentrations and low 129 I/I ratios of this subgroup, as well as a moderate depletion of deuterium relative to 18 O, suggest that these waters were not significantly altered since the time when diagenetic reactions occurred in the deepest portion of the basin. A third subgroup, with higher δD and δ 18 O values as well as higher 129 I/I ratios, extends roughly west to east at the New Mexico–Colorado state line and corresponds to a region of extensive fracturing of the coalbeds. In this case, the higher 129 I/I ratios are probably due to contributions of fissiogenic 129 I through fracture flow, perhaps from deeper formation waters. Our results do not support models of subsequent basin-wide groundwater migration in the Fruitland Formation. The combined use of 129 I and 36 Cl with stable isotope studies provides valuable information as to the hydrologic history of coalbed methane deposits, as well as their potential for commercial exploitation.

Geochemical Cycling of Iodine Species in Soils

This chapter applies new analytical techniques to study the content and speciation of stable iodine in representative surface soils, and sorption and transport behavior of iodine species (iodide, iodate, and 4-iodoaniline) in sediments collected at numerous nuclear facilities in the United States, where anthropogenic 129 I from prior nuclear fuel processing activities poses an environmental risk. Different iodine species exhibit very different sorption and transport behavior in geologic samples. Sorption of iodate is consistently greater than that of iodide, while the transport of organoiodine (exemplified by 4-iodoaniline in this study) is quite limited, and related to the amount of organic matter in the sample. The physical and chemical processes affecting iodine transport include iodate reduction, irreversible retention or mass loss, and rate-limited and nonlinear sorption of iodine. Various iodine species in different proportions exist in soil and sediments. Organically bound iodine, with limited solubility and mobility, commonly comprises the major proportion of total iodine; it is well correlated with total organic matter, sesquioxides, and clay content. With a shorter contact time than stable iodine, anthropogenic 129 I will likely have a higher mobility when in an inorganic form. However, 129 I will experience a similar speciation process as stable iodine and will eventually be retained strongly, with organoiodine as the dominant species.

High-resolution simulation of basin-scale nitrate transport considering aquifer system heterogeneity

Nitrate contamination presents a growing threat to many groundwater basins relied upon for drinking water. This study combines geostatistical techniques, parallel computing of fl ow simulation, and particle tracking to develop realistic nitrate loading and transport scenarios. The simulation scenarios are patterned after the rapidly urbanizing Llagas groundwater subbasin in the south San Francisco Bay area of California. In the Llagas subbasin, groundwater is the sole municipal water supply. A key component of this study is the development of a highly resolved model of the heterogeneity in the aquifer system using a new geostatistical technique for simulating hydrofacies architecture that can incorporate uncertain or “soft” data, such as well driller logs. Numerical simulations of nitrate transport indicate the degree to which a heterogeneous conceptual model can account for dispersion relative to a conventional homogeneous model assuming typical dispersivity coeffi cients. The heterogeneous model transport results are found to be consistent with observed nitrate contamination patterns and depth distribution as well as groundwater-age trends with depth. The model provides a realistic test-bed for prediction of future nitrate concentrations, including the time frame for potential nitrate impacts to deep wells, given that geochemical data indicate that denitrifi cation is not likely to occur.

A membrane inlet mass spectrometry system for noble gases at natural abundances in gas and water samples

RATIONALE Noble gases dissolved in groundwater can reveal paleotemperatures, recharge conditions, and precise travel times. The collection and analysis of noble gas samples are cumbersome, involving noble gas purification, cryogenic separation and static mass spectrometry. A quicker and more efficient sample analysis method is required for introduced tracer studies and laboratory experiments. METHODS A Noble Gas Membrane Inlet Mass Spectrometry (NG-MIMS) system was developed to measure noble gases at natural abundances in gas and water samples. The NG-MIMS system consists of a membrane inlet, a dry-ice water trap, a carbon-dioxide trap, two getters, a gate valve, a turbomolecular pump and a quadrupole mass spectrometer equipped with an electron multiplier. Noble gases isotopes (4)He, (22)Ne, (38)Ar, (84)Kr and (132)Xe are measured every 10 s. RESULTS The NG-MIMS system can reproduce measurements made on a traditional noble gas mass spectrometer system with precisions of 2%, 8%, 1%, 1% and 3% for He, Ne, Ar, Kr and Xe, respectively. Noble gas concentrations measured in an artificial recharge pond were used to monitor an introduced xenon tracer and to reconstruct temperature variations to within 2 °C. Additional experiments demonstrated the capability to measure noble gases in gas and in water samples, in real time. CONCLUSIONS The NG-MIMS system is capable of providing analyses sufficiently accurate and precise for introduced noble gas tracers at managed aquifer recharge facilities, groundwater fingerprinting based on excess air and noble gas recharge temperature, and field and laboratory studies investigating ebullition and diffusive exchange.

Movement of Water Infiltrated from a Recharge Basin to Wells

Local surface water and stormflow were infiltrated intermittently from a 40-ha basin between September 2003 and September 2007 to determine the feasibility of recharging alluvial aquifers pumped for public supply, near Stockton, California. Infiltration of water produced a pressure response that propagated through unconsolidated alluvial-fan deposits to 125 m below land surface (bls) in 5 d and through deeper, more consolidated alluvial deposits to 194 m bls in 25 d, resulting in increased water levels in nearby monitoring wells. The top of the saturated zone near the basin fluctuates seasonally from depths of about 15 to 20 m. Since the start of recharge, water infiltrated from the basin has reached depths as great as 165 m bls. On the basis of sulfur hexafluoride tracer test data, basin water moved downward through the saturated alluvial deposits until reaching more permeable zones about 110 m bls. Once reaching these permeable zones, water moved rapidly to nearby pumping wells at rates as high as 13 m/d. Flow to wells through highly permeable material was confirmed on the basis of flowmeter logging, and simulated numerically using a two-dimensional radial groundwater flow model. Arsenic concentrations increased slightly as a result of recharge from 2 to 6 µg/L immediately below the basin. Although few water-quality issues were identified during sample collection, high groundwater velocities and short travel times to nearby wells may have implications for groundwater management at this and at other sites in heterogeneous alluvial aquifers.

Simulation of Nitrate Biogeochemistry and Reactive Transport in a California Groundwater Basin

Nitrate is the number one drinking water contaminant in the United States. It is pervasive in surface and groundwater systems, and its principal anthropogenic sources have increased dramatically in the last 50 years. In California alone, one third of the public drinking-water wells has been lost since 1988 and nitrate contamination is the most common reason for abandonment. Effective nitrate management in groundwater is complicated by uncertainties related to multiple point and non-point sources, hydrogeologic complexity, geochemical reactivity, and quantification of denitrification processes. In this paper, we review an integrated experimental and simulation-based framework being developed to study the fate of nitrate in a 25 km-long groundwater subbasin south of San Jose, California, a historically agricultural area now undergoing rapid urbanization with increasing demands for groundwater. The modeling approach is driven by a need to integrate new and archival data that support the hypothesis that nitrate fate and transport at the basin scale is intricately related to hydrostratigraphic complexity, variability of flow paths and groundwater residence times, microbial activity, and multiple geochemical reaction mechanisms. This study synthesizes these disparate and multi-scale data into a three-dimensional and highly resolved reactive transport modeling framework.

Importance of river water recharge to the San Joaquin Valley groundwater system

Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA, USA Department of Earth & Environmental Sciences, California State University, East Bay, Hayward, CA, USA Correspondence Ate Visser, Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, CA 94551‐0808, USA. Email: visser3@llnl.gov Funding information California State Water Resources Control Board; LLNL LDRD, Grant/Award Number: 15‐ERD‐042

Assessing Contamination Susceptibility at Public Water Supply Wells in California

The Ambient Groundwater Monitoring and Assessment program, sponsored by the California State Water Resources Control Board, uses a probabilistic approach to assess the vulnerability of public water supply wells to contamination by anthropogenic compounds. Sources of contamination to groundwater occur near the earth’s surface, and have been present mostly since WWII. Therefore, wells that receive water that has recharged in the recent past are more likely to intercept contaminants that have been transported by advection. The parameters that the study uses to rank wells according to vulnerability are groundwater age dates (using the tritium/helium method), stable isotopes of the water molecule (for water source determination), and analysis of low level Volatile Organic Compounds (VOCs). Results of a pilot project in which 500 public water supply wells were tested for vulnerability will be presented. Basins sampled for the study include the Livermore Valley, Orange County Basin, Santa Clara Valley, and the Sacramento Basin. Methyl-tertiary-Butyl Ether (MTBE) may be a useful time marker in groundwater basins, with water recharged after the 1980s showing traces of MTBE. Low-level detections of other VOCs such as TCE and PCE can give an early warning of a contaminant plume. When employed on a basin-scale, groundwater ages are an