Suzanne S. Paschke

Assessment of surface water chloride and conductivity trends in areas of unconventional oil and gas development—Why existing national data sets cannot tell us what we would like to know

Heightened concern regarding the potential effects of unconventional oil and gas development on regional water quality has emerged, but the few studies on this topic are limited in geographic scope. Here we evaluate the potential utility of national and publicly available water-quality data sets for addressing questions regarding unconventional oil and gas development. We used existing U.S. Geological Survey and U.S. Environmental Protection Agency data sets to increase understanding of the spatial distribution of unconventional oil and gas development in the U.S. and broadly assess surface water quality trends in these areas. Based on sample size limitations, we were able to estimate trends in specific conductance (SC) and chloride (Cl−) from 1970 to 2010 in 16% (n = 155) of the watersheds with unconventional oil and gas resources. We assessed these trends relative to spatiotemporal distributions of hydraulically fractured wells. Results from this limited analysis suggest no consistent and widespread trends in surface water quality for SC and Cl− in areas with increasing unconventional oil and gas development and highlight limitations of existing national databases for addressing questions regarding unconventional oil and gas development and water quality.

Near-decadal changes in nitrate and pesticide concentrations in the South Platte River alluvial aquifer, 1993-2004

The lower South Platte River basin of Colorado and Nebraska is an area of intense agriculture supported by surface-water diversions from the river and ground-water pumping from a valley-fill alluvial aquifer. Two well networks consisting of 45 wells installed in the South Platte alluvial aquifer were sampled in the early 1990s and again in the early 2000s to examine near-decadal ground-water quality changes in irrigated agricultural areas. Ground-water age generally increases and dissolved-oxygen content decreases with distance along flow paths and with depth below the water table, and denitrification is an important natural mitigation mechanism for nitrate in downgradient areas. Ground-water travel time from upland areas to the river ranges from 12 to 31 yr on the basis of apparent ground-water ages. Ground-water nitrate concentrations for agricultural land-use wells increased significantly for oxidized samples over the decade, and nitrogen isotope ratios for oxidized samples indicate synthetic fertilizer as the predominant nitrate source. Ground-water concentrations of atrazine, DEA, and prometon decreased significantly. The decrease in pesticide concentrations and a significant increase in the ratio of DEA to atrazine suggest decreases in pesticide concentrations are likely caused by local decreases in application rates and/or degradation processes and that atrazine degradation is promoted by oxidizing conditions. The difference between results for oxidizing and nitrate-reducing conditions indicates redox state is an important variable to consider when evaluating ground-water quality trends for redox-sensitive constituents such as nitrate and pesticides in the South Platte alluvial aquifer.

MODFLOW2000 model used to simulate the groundwater flow of the Denver Basin Aquifer System, Colorado

A three-dimensional groundwater flow model (MODFLOW2000) of the Denver Basin bedrock aquifer system and overlying alluvial aquifer was developed to provide quantitative estimates of groundwater flow conditions and provide a useful tool for managers to analyze temporal changes to the hydrologic system in response to changing climatic conditions and future groundwater development. In 2004, the U.S. Geological Survey (USGS) initiated large-scale regional studies to provide updated assessments of groundwater availability in important principal aquifers across the United States, including the Denver Basin. The Denver Basin groundwater flow model includes several enhancements over previous modeling efforts because of the availability of additional data, improved modeling capabilities, and advanced computer technology. Additional data available include updated geologic mapping, additional geophysical logs, water-level, streamflow, precipitation, and irrigation data collected since previous studies; and updated estimates of pumping from Denver Basin bedrock and alluvial aquifers. Modeling capabilities and computer technology also have advanced such that additional features, hydrologic processes, and numerical techniques are included in the current model that were not possible in previous models. The Denver Basin groundwater flow model represents regional time-varying (transient) conditions prior to 1880 through 2003. The model was calibrated by primarily adjusting hydraulic conductivity and recharge parameters until a best fit was obtained between observed and simulated transient hydraulic heads and flows using PEST. The calibrated model was used to estimate the hydrologic system response to two pumping scenarios for the period 2004 through 2053. This USGS data release contains all of the input and output files for the simulation and calibration described in the associated model documentation report (https://pubs.er.usgs.gov/publication/pp1770).