Geology topics

David T. Long

Publications and source records attributed to David T. Long.

6 recordsLinked to original sources

USGS2016

Linking silicate weathering to riverine geochemistry—A case study from a mountainous tropical setting in west-central Panama

Chemical analyses from 71 watersheds across an ∼450 km transect in west-central Panama provide insight into controls on weathering and rates of chemical denudation and CO 2 consumption across an igneous arc terrain in the tropics. Stream and river compositions across this region of Panama are generally dilute, having a total dissolved solute value = 118 ± 91 mg/L, with bicarbonate and silica being the predominant dissolved species. Solute, stable isotope, and radiogenic isotope compositions are consistent with dissolution of igneous rocks present in Panama by meteoric precipitation, with geochemical signatures of rivers largely acquired in their upstream regions. Comparison of a headwater basin with its entire watershed observed considerably more runoff production from the high-elevation upstream portion of the catchment than in its much more spatially extensive downstream region. Rock alteration profiles document that weathering proceeds primarily by dissolution of feldspar and pyroxene, with base cations effectively leached in the following sequence: Na > Ca > Mg > K. Control on water chemistry by bedrock lithology is indicated through a linking of elevated ([Na + K]/[Ca + Mg]) ratios in waters to a high proportion of catchment area silicic bedrock and low ratios to mafic bedrock. Sr-isotope ratios are dominated by basement-derived Sr, with only very minor, if any, contribution from other sources. Cation weathering of Ca sil + Mg sil + Na + K spans about an order in magnitude, from 3 to 32 tons/km 2 /yr. Strong positive correlations of chemical denudation and CO 2 consumption are observed with precipitation, mean watershed elevation, extent of land surface forest cover, and physical erosion rate.

Chagras and Pacora watersheds↗

USGS2003

Identifying relationships between baseflow geochemistry and land use with synoptic sampling and R-mode factor analysis

The relationship between land use and stream chemistry is often explored through synoptic sampling of rivers at baseflow conditions. However, baseflow chemistry is likely to vary temporally and spatially with land use. The purpose of our study is to examine the usefulness of the synoptic sampling approach for identifying the relationship between complex land use configurations and stream water quality. This study compares biogeochemical data from three synoptic sampling events representing the temporal variability of baseflow chemistry and land use using R - mode factor analysis . Separate R - mode factor analyses of the data from individual sampling events yielded only two consistent factors. Agricultural activity was associated with elevated levels of Ca 2+ , Mg 2+ , alkalinity, and frequently K + , SO 2- 4 , and NO - 3 . Urban areas were associated with higher concentrations of Na + , K + , and Cl - . Other retained factors were not consistent among sampling events, and some factors were difficult to interpret in the context of biogeochemical sources and processes. When all data were combined, further associations were revealed such as an inverse relationship between the proportion of wetlands and stream nitrate concentrations. We also found that barren lands were associated with elevated sulfate levels. This research suggests that an individual sampling event is unlikely to characterize adequately the complex processes controlling interactions between land use and stream chemistry. Combining data collected over two years during three synoptic sampling events appears to enhance our ability to understand processes linking stream chemistry and land use.

Michigan↗

USGS2002

Evaluating behavior of oxygen, nitrate, and sulfate during recharge and quantifying reduction rates in a contaminated aquifer

This study evaluates the biogeochemical changes that occur when recharge water comes in contact with a reduced aquifer. It specifically addresses (1) which reactions occur in situ, (2) the order in which these reactions will occur if terminal electron acceptors (TEAs) are introduced simultaneously, (3) the rates of these reactions, and (4) the roles of the aqueous and solid-phase portions of the aquifer. Recharge events of waters containing various combinations of O 2 , NO 3 , and SO 4 were simulated at a shallow sandy aquifer contaminated with waste fuels and chlorinated solvents using modified push−pull tests to quantify rates. In situ rate constants for aerobic respiration (14.4 day - 1 ), denitrification (5.04−7.44 day - 1 ), and sulfate reduction (4.32−6.48 day - 1 ) were estimated. Results show that when introduced together, NO 3 and SO 4 can be consumed simultaneously at similar rates. To distinguish the role of aqueous phase from that of the solid phase of the aquifer, groundwater was extracted, amended with NO 3 and SO 4 , and monitored over time. Results indicate that neither NO 3 nor SO 4 was reduced during the course of the aqueous-phase study, suggesting that NO 3 and SO 4 can behave conservatively in highly reduced water. It is clear that sediments and their associated microbial communities are important in driving redox reactions.

Environmental Science & Technology↗

USGS2000

Temporal variations in parameters reflecting terminal-electron-accepting processes in an aquifer contaminated with waste fuel and chlorinated solvents

A fundamental issue in aquifer biogeochemistry is the means by which solute transport, geochemical processes, and microbiological activity combine to produce spatial and temporal variations in redox zonation. In this paper, we describe the temporal variability of TEAP conditions in shallow groundwater contaminated with both waste fuel and chlorinated solvents. TEAP parameters (including methane, dissolved iron, and dissolved hydrogen) were measured to characterize the contaminant plume over a 3-year period. We observed that concentrations of TEAP parameters changed on different time scales and appear to be related, in part, to recharge events. Changes in all TEAP parameters were observed on short time scales (months), and over a longer 3-year period. The results indicate that (1) interpretations of TEAP conditions in aquifers contaminated with a variety of organic chemicals, such as those with petroleum hydrocarbons and chlorinated solvents, must consider additional hydrogen-consuming reactions (e.g., dehalogenation); (2) interpretations must consider the roles of both in situ (at the sampling point) biogeochemical and solute transport processes; and (3) determinations of microbial communities are often necessary to confirm the interpretations made from geochemical and hydrogeological measurements on these processes.

Michigan↗

USGS1996

Selected geochemical characteristics of ground water from the Saginaw aquifer in the central Lower Peninsula of Michigan

Chemical and stable-isotope data of water from wells completed in the Saginaw aquifer in the central Lower Peninsula of Michigan were used to prepare maps that show areal variation of δ 18 O; distribution of dissolved solids, dissolved chloride, dissolved iron, dissolved sulfate; and distribution of hydrochemical facies. Delta oxygen-18 values indicate the presence of modern meteoric water (δ 18 O approximately -10 parts per thousand) and glacial-age meteoric water, which is isotopically light (δ 18 O less than -15 parts per thousand). Isotopically light ground water is present in the Saginaw Bay Area in the eastern part of the study area. Dissolved-solids concentration ranges from 41 to 92,300 milligrams per liter, and dissolved-chloride concentrations range from less than 1 to 55,000 milligrams per liter. Dissolved-solids and dissolved-chloride concentrations increase toward Saginaw Bay. Dissolved-iron and dissolved-sulfate concentration ranges from 0.01 to 7.80 and 0.2 to 3,500 milligrams per liter, respectively. Most ground water from the Saginaw aquifer is classified as calcium bicarbonate, calcium sulfate, or sodium chloride.

Michigan↗

USGS1996

Selected geochemical characteristics of ground water from the Marshall aquifer in the central Lower Peninsula of Michigan

Chemistry and stable-isotope data for water from wells completed in the Marshall aquifer within the Michigan Basin were used to prepare maps that show area! variations of δ 18 O; distribution of dissolved solids, dissolved chloride, dissolved iron, and dissolved sulfate; and distribution of hydrochemical facies. Delta oxygen-18 values indicate the presence of modern meteoric water ( δ 18 O approximately -10 parts per mil) as well as isotopically light meteoric water ( δ 18 O less than -15 parts per mil). Isotopically light ground water is present in the Michigan Lowland. Dissolved-solids concentrations range from 273 to 412,000 milligrams per liter, and dissolved-chloride concentrations range from less than 10 to greater than 250,000 milligrams per liter. Dissolved-solids and dissolved-chloride concentrations increase toward the center of the study area. Dissolved-iron concentrations which range from less than 0.002 to 100 milligrams per liter, are low in the subcrop areas, and increase toward the center of the study area. Dissolved-sulfate concentrations range from 4.5 to 3,500 milligrams per liter and generally increase then decrease from the subcrop area toward the center of the study area. Most ground water in the Marshall aquifer is classified as either a calcium bicarbonate or a sodium chloride type.

Michigan↗