Geology topics

M.A. Thomas

Publications and source records attributed to M.A. Thomas.

4 recordsLinked to original sources

USGS2022

Mercury contamination from historic mining in water and sediment, Guadalupe River and San Francisco Bay, California

The New Almaden mercury mines in California (USA), which collectively represent the largest historic producers of mercury in North America, are a persistent source of mercury contamination to the San Francisco Bay estuary. An estimate based on total mercury concentration (HgTOT) and provisional stream flow data measured at a gauging station in the Guadalupe River during base flow conditions yields a base flow flux of 30 g of mercury for the month of October 2000. In contrast to this base flow estimate, one 2-day rain event in October 2000 resulted in a flux of 22 g of mercury past this site. An estimate of mercury transport from the entire Guadalupe River watershed based on a sediment transport model and our measured suspended particulate HgTOT (0.5-4 ??g g-1) results in a total of 4-30 kg year-1 transported to the southern reach of the estuary. Sediments in the southern reach have lower HgTOT (most ??? 0.4 ??g g-1 dry wt) and monomethyl-mercury (MMHg, c. 1 ng g-1 dry wt) concentrations than those in the Guadalupe River (HgTOT, 0.41-33 ??g g-1 dry wt; MMHg, 1-10 ng g-1 dry wt). Because the most elevated methylmercury concentrations (8-12 ng g-1 dry wt) were found in sediments deposited immediately upstream of hydraulic structures (e.g. diversion dams and weirs) within the river, it is proposed that such physical structures may represent important zones of MMHg production and fluxes to San Francisco Bay.

Geochemistry: Exploration, Environment, Analysis↗

USGS2009

Distinguishing iron-reducing from sulfate-reducing conditions

Ground water systems dominated by iron‐ or sulfate‐reducing conditions may be distinguished by observing concentrations of dissolved iron (Fe 2+ ) and sulfide (sum of H 2 S, HS − , and S = species and denoted here as “H 2 S”). This approach is based on the observation that concentrations of Fe 2+ and H 2 S in ground water systems tend to be inversely related according to a hyperbolic function. That is, when Fe 2+ concentrations are high, H 2 S concentrations tend to be low and vice versa. This relation partly reflects the rapid reaction kinetics of Fe 2+ with H 2 S to produce relatively insoluble ferrous sulfides (FeS). This relation also reflects competition for organic substrates between the iron‐ and the sulfate‐reducing microorganisms that catalyze the production of Fe 2+ and H 2 S. These solubility and microbial constraints operate in tandem, resulting in the observed hyperbolic relation between Fe 2+ and H 2 S concentrations. Concentrations of redox indicators, including dissolved hydrogen (H 2 ) measured in a shallow aquifer in Hanahan, South Carolina, suggest that if the Fe 2+ /H 2 S mass ratio (units of mg/L) exceeded 10, the screened interval being tapped was consistently iron reducing (H 2 ∼0.2 to 0.8 nM). Conversely, if the Fe 2+ /H 2 S ratio was less than 0.30, consistent sulfate‐reducing (H 2 ∼1 to 5 nM) conditions were observed over time. Concomitantly high Fe 2+ and H 2 S concentrations were associated with H 2 concentrations that varied between 0.2 and 5.0 nM over time, suggesting mixing of water from adjacent iron‐ and sulfate‐reducing zones or concomitant iron and sulfate reduction under nonelectron donor–limited conditions. These observations suggest that Fe 2+ /H 2 S mass ratios may provide useful information concerning the occurrence and distribution of iron and sulfate reduction in ground water systems.

Ground Water↗

USGS2009

Combining particle-tracking and geochemical data to assess public supply well vulnerability to arsenic and uranium

Flow-model particle-tracking results and geochemical data from seven study areas across the United States were analyzed using three statistical methods to test the hypothesis that these variables can successfully be used to assess public supply well vulnerability to arsenic and uranium. Principal components analysis indicated that arsenic and uranium concentrations were associated with particle-tracking variables that simulate time of travel and water fluxes through aquifer systems and also through specific redox and pH zones within aquifers. Time-of-travel variables are important because many geochemical reactions are kinetically limited, and geochemical zonation can account for different modes of mobilization and fate. Spearman correlation analysis established statistical significance for correlations of arsenic and uranium concentrations with variables derived using the particle-tracking routines. Correlations between uranium concentrations and particle-tracking variables were generally strongest for variables computed for distinct redox zones. Classification tree analysis on arsenic concentrations yielded a quantitative categorical model using time-of-travel variables and solid-phase-arsenic concentrations. The classification tree model accuracy on the learning data subset was 70%, and on the testing data subset, 79%, demonstrating one application in which particle-tracking variables can be used predictively in a quantitative screening-level assessment of public supply well vulnerability. Ground-water management actions that are based on avoidance of young ground water, reflecting the premise that young ground water is more vulnerable to anthropogenic contaminants than is old ground water, may inadvertently lead to increased vulnerability to natural contaminants due to the tendency for concentrations of many natural contaminants to increase with increasing ground-water residence time.

Journal of Hydrology↗

USGS2007

The effect of residential development on ground-water quality near Detroit, Michigan

Two water-quality studies were done on the outskirts of the Detroit metropolitan area to determine how recent residential development has affected ground-water quality. Pairs of monitor and domestic wells were sampled in areas where residential land use overlies glacial outwash deposits. Young, shallow waters had significantly higher median concentrations of nitrate, chloride, and dissolved solids than older, deeper waters. Analysis of chloride/bromide ratios indicates that elevated salinities are due to human activities rather than natural factors, such as upward migration of brine. Trace concentrations of volatile organic compounds were detected in samples from 97 percent of the monitor wells. Pesticides were detected infrequently even though they are routinely applied to lawns and roadways in the study area. The greatest influence on ground-water quality appears to be from septic-system effluent (domestic sewage, household solvents, water-softener backwash) and infiltration of stormwater runoff from paved surfaces (road salt, fuel residue). No health-related drinking-water standards were exceeded in samples from domestic wells. However, the effects of human activities are apparent in 76 percent of young waters, and at depths far below 25 feet, which is the current minimum well-depth requirement.Two water-quality studies were done on the outskirts of the Detroit metropolitan area to determine how recent residential development has affected ground-water quality. Pairs of monitor and domestic wells were sampled in areas where residential land use overlies glacial outwash deposits. Young, shallow waters had significantly higher median concentrations of nitrate, chloride, and dissolved solids than older, deeper waters. Analysis of chloride/bromide ratios indicates that elevated salinities are due to human activities rather than natural factors, such as upward migration of brine. Trace concentrations of volatile organic compounds were detected in samples from 97 percent of the monitor wells. Pesticides were detected infrequently even though they are routinely applied to lawns and roadways in the study area. The greatest influence on ground-water quality appears to be from septic-system effluent (domestic sewage, household solvents, water-softener backwash) and infiltration of stormwater runoff from paved surfaces (road salt, fuel residue). No health-related drinking-water standards were exceeded in samples from domestic wells. However, the effects of human activities are apparent in 76 percent of young waters, and at depths far below 25 feet, which is the current minimum well-depth requirement.

Michigan↗