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P.B. McMahon

Publications and source records attributed to P.B. McMahon.

At least 37 records · Page 2Linked to original sources
USGS2006

Storage and transit time of chemicals in thick unsaturated zones under rangeland and irrigated cropland, High Plains, United States

In 2000-2002, three rangeland and six irrigated sites were instrumented to assess the storage and transit time of chemicals in thick (15 to 50 m) unsaturated zones (UZ) in the High Plains. These processes are likely to influence relations between land use and groundwater quality, yet they have not been documented systematically in the High Plains. Land use and climate were important controls on the size of subsoil chloride, nitrate, and pesticide compound reservoirs. The reservoirs under irrigated cropland generally were larger than those under rangeland because more chemicals were applied to cropland than to rangeland. In some cases, chloride and nitrate reservoirs under rangeland were larger than those under cropland, presumably because of long‐term evaporative concentration near the base of the root zone. Natural salts mobilized by irrigation return flow accounted for as much as 60 and 80% of the nitrate and chloride reservoirs, respectively, under some cropland, as indicated by detailed chemical profiles and isotopic tracers ( 15 N, 18 O in nitrate and 2 H, 3 H, 18 O in water). Advective chemical transit times in the UZ under cropland ranged from about 50 to 375 years, longer than any of the instrumented fields had been irrigated, yet agrichemicals were detected at the water table at four of the six sites. The data provide evidence for the existence of slow and fast paths for water movement in the UZ, with larger subsoil chemical reservoirs occurring in areas dominated by slow paths. Implications of these findings with respect to water quality in the aquifer are significant because they indicate that the amount of chemical mass reaching the aquifer could increase with time as chemicals that still reside under irrigated fields reach the water table.

Water Resources Research↗
USGS2005

Hydraulic and geochemical performance of a permeable reactive barrier containing zero-valent iron, Denver Federal Center

The hydraulic and geochemical performance of a 366 m long permeable reactive barrier (PRB) at the Denver Federal Center, Denver, Colorado, was evaluated. The funnel and gate system, which was installed in 1996 to intercept and remediate ground water contaminated with chlorinated aliphatic hydrocarbons (CAHs), contained four 12.2 m wide gates filled with zero‐valent iron. Ground water mounding on the upgradient side of the PRB resulted in a tenfold increase in the hydraulic gradient and ground water velocity through the gates compared to areas of the aquifer unaffected by the PRB. Water balance calculations for April 1997 indicate that about 75 % of the ground water moving toward the PRB from upgradient areas moved through the gates. The rest of the water either accumulated on the upgradient side of the PRB or bypassed the PRB. Chemical data from monitoring wells screened down‐gradient, beneath, and at the ends of the PRB indicate that contaminants had not bypassed the PRB, except in a few isolated areas. Greater than 99 % of the CAH mass entering the gates was retained by the iron. Fifty‐one percent of the CAH carbon entering one gate was accounted for in dissolved C 1 and C 2 hydrocarbons, primarily ethane and ethene, which indicates that CAHs may adsorb to the iron prior to being dehalogenated. Treated water exiting the gates displaced contaminated ground water at a distance of at least 3 m downgradient from the PRB by the end of 1997. Measurements of dissolved inorganic ions in one gate indicate that calcite and siderite precipitation in the gate could reduce gate porosity by about 0.35 % per year. Results from this study indicate that funnel and gate systems containing zero‐valent iron can effectively treat ground water contaminated with CAHs. However, the hydrologic impacts of the PRB on the flow system need to be fully understood to prevent contaminants from bypassing the PRB.

Colorado↗
USGS2004

Geochemistry, radiocarbon ages, and paleorecharge conditions along a transect in the central High Plains aquifer, southwestern Kansas, USA

Water samples from short-screen monitoring wells installed along a 90-km transect in southwestern Kansas were analyzed for major ions, trace elements, isotopes (H, B, C, N, O, S, Sr), and dissolved gases (He, Ne, N 2 , Ar, O 2 , CH 4 ) to evaluate the geochemistry, radiocarbon ages, and paleorecharge conditions in the unconfined central High Plains aquifer. The primary reactions controlling water chemistry were dedolomitization, cation exchange, feldspar weathering, and O 2 reduction and denitrification. Radiocarbon ages adjusted for C mass transfers ranged from <2.6 ka ( 14 C) B.P. near the water table to 12.8 ± 0.9 ka ( 14 C) B.P. at the base of the aquifer, indicating the unconfined central High Plains aquifer contained a stratified sequence of ground water spanning Holocene time. A cross-sectional model of steady-state ground-water flow, calibrated using radiocarbon ages, is consistent with recharge rates ranging from 0.8 mm/a in areas overlain by loess to 8 mm/a in areas overlain by dune sand. Paleorecharge temperatures ranged from an average of 15.2 ± 0.7 °C for the most recently recharged waters to 11.6 ± 0.4 °C for the oldest waters. The temperature difference between Early and Late Holocene recharge was estimated to be 2.4 ± 0.7 °C, after taking into account variable recharge elevations. Nitrogen isotope data indicate NO 3 in paleorecharge (average concentration=193 μM) was derived from a relatively uniform source such as soil N, whereas NO 3 in recent recharge (average concentration=885 μM) contained N from varying proportions of fertilizer, manure, and soil N. Deep water samples contained components of N 2 derived from atmospheric, denitrification, and deep natural gas sources. Denitrification rates in the aquifer were slow (5 ± 2× 10 −3 μmol N L −1 a −1 ), indicating this process would require >10 ka to reduce the average NO 3 concentration in recent recharge to the Holocene background concentration.

Kansas↗
USGS2004

Vertical gradients in water chemistry and age in the southern High Plains Aquifer, Texas, 2002

The southern High Plains aquifer is the primary source of water used for domestic, industrial, and irrigation purposes in parts of New Mexico and Texas. Despite the aquifer's importance to the overall economy of the southern High Plains, fundamental ground-water characteristics, such as vertical gradients in water chemistry and age, remain poorly defined. As part of the U.S. Geological Survey's National Water-Quality Assessment Program, water samples from nested, short-screen monitoring wells installed in the southern High Plains aquifer at two locations (Castro and Hale Counties, Texas) were analyzed for field parameters, major ions, nutrients, trace elements, dissolved organic carbon, pesticides, stable and radioactive isotopes, and dissolved gases to evaluate vertical gradients in water chemistry and age in the aquifer. Tritium measurements indicate that recent (post-1953) recharge was present near the water table and that deeper water was recharged before 1953. Concentrations of dissolved oxygen were largest (2.6 to 5.6 milligrams per liter) at the water table and decreased with depth below the water table. The smallest concentrations were less than 0.5 milligram per liter. The largest major-ion concentrations generally were detected at the water table because of the effects of overlying agricultural activities, as indicated by postbomb tritium concentrations and elevated nitrate and pesticide concentrations at the water table. Below the zone of agricultural influence, major-ion concentrations exhibited small increases with depth and distance along flow paths because of rock/water interactions and mixing with water from the underlying aquifer in rocks of Cretaceous age. The concentration increases primarily were accounted for by dissolved sodium, bicarbonate, chloride, and sulfate. Nitrite plus nitrate concentrations at the water table were 2.0 to 6.1 milligrams per liter as nitrogen, and concentrations substantially decreased with depth in the aquifer to a maximum concentration of 0.55 milligram per liter as nitrogen. Dissolved-gas and nitrogen-isotope data from the deep wells in Castro County indicate that denitrification occurred in the aquifer, removing 74 to more than 97 percent of the nitrate originally present in recharge. There was no evidence of denitrification in the deep part of the aquifer in Hale County. After correcting for denitrification effects, the background concentration of nitrate in water recharged before 1953 ranged from 0.4 to 3.2 milligrams per liter as nitrogen, with an average of 1.6 milligrams per liter as nitrogen. The d15N composition of background nitrate at the time of recharge was estimated to range from 9.6 to 12.3 per mil. Mass-balance models indicate that the decreases in dissolved oxygen and nitrate concentrations and small increases in major-ion concentrations along flow paths can be accounted for by small amounts of silicate-mineral and calcite dissolution; SiO2, goethite, and clay-mineral precipitation; organic-carbon and pyrite oxidation; denitrification; and cation exchange. Mass-balance models for some wells also required mixing with water from the underlying aquifer in rocks of Cretaceous age to achieve mole and isotope balances. Carbon mass transfers identified in the models were used to adjust radiocarbon ages of water samples recharged before 1953. Adjusted radiocarbon ages ranged from less than 1,000 to 9,000 carbon-14 years before present. Radiocarbon ages were more sensitive to uncertainties in the carbon-14 content of recharge than uncertainties in carbon mass transfers, leading to 1-sigma uncertainties of about ?2,000 years in the adjusted ages. Despite these relatively large uncertainties in adjusted radiocarbon ages, it appears that deep water in the aquifer was considerably older (at least 1,000 years) than water near the water table. There was essentially no change in ground-water age with depth in deeper parts of the aquifer, indicating that water in that

Texas↗
USGS2002

The High Plains Aquifer, USA: Groundwater development and sustainability

The High Plains Aquifer, located in the United States, is one of the largest freshwater aquifers in the world and is threatened by continued decline in water levels and deteriorating water quality. Understanding the physical and cultural features of this area is essential to assessing the factors that affect this groundwater resource. About 27% of the irrigated land in the United States overlies this aquifer, which yields about 30% of the nation's groundwater used for irrigation of crops including wheat, corn, sorghum, cotton and alfalfa. In addition, the aquifer provides drinking water to 82% of the 2.3 million people who live within the aquifer boundary. The High Plains Aquifer has been significantly impacted by human activities. Groundwater withdrawals from the aquifer exceed recharge in many areas, resulting in substantial declines in groundwater level. Residents once believed that the aquifer was an unlimited resource of high-quality water, but they now face the prospect that much of the water may be gone in the near future. Also, agricultural chemicals are affecting the groundwater quality. Increasing concentrations of nitrate and salinity can first impair the use of the water for public supply and then affect its suitability for irrigation. A variety of technical and institutional measures are currently being planned and implemented across the aquifer area in an attempt to sustain this groundwater resource for future generations. However, because groundwater withdrawals remain high and water quality impairments are becoming more commonplace, the sustainability of the High Plains Aquifer is uncertain.

Colorado, Kansas, Nebraska, New Mexico, Oklahoma, ↗
USGS2001

Aquifer/aquitard interfaces: Mixing zones that enhance biogeochemical reactions

Several important biogeochemical reactions are known to occur near the interface between aquifer and aquitard sediments. These reactions include O2 reduction; denitrification; and Fe3+, SO42-, and CO2 (methanogenesis) reduction. In some settings, these reactions occur on the aquitard side of the interface as electron acceptors move from the aquifer into the electron-donor-enriched aquitard. In other settings, these reactions occur on the aquifer side of the interface as electron donors move from the aquitard into the electron-acceptor-enriched, or microorganism-enriched, aquifer. Thus, the aquifer/aquitard interface represents a mixing zone capable of supporting greater microbial activity than either hydrogeologic unit alone. The extent to which biogeochemical reactions proceed in the mixing zone and the width of the mixing zone depend on several factors, including the abundance and solubility of electron acceptors and donors on either side of the interface and the rate at which electron acceptors and donors react and move across the interface. Biogeochemical reactions near the aquifer/aquitard interface can have a substantial influence on the chemistry of water in aquifers and on the chemistry of sediments near the interface.

Hydrogeology Journal↗
USGS2000

Occurrence of nitrous oxide in the central High Plains aquifer, 1999

Nitrogen-enriched groundwater has been proposed as an important anthropogenic source of atmospheric nitrous oxide (N2O), yet few measurements of N2O in large aquifer systems have been made. Concentrations of N2O in water samples collected from the 124 000 km2 central High Plains aquifer in 1999 ranged from < 1 to 940 nM, with a median concentration of 29 nM (n = 123). Eighty percent of the N20 concentrations exceeded the aqueous concentration expected from equilibration with atmospheric N2O. Measurements of N2O, NO3-, and 3H in unsaturated-zone sediments, recently recharged groundwater, and older groundwater indicate that concentrations of N2O in groundwater increased over time and will likely continue to increase in the future as N-enriched water recharges the aquifer. Large concentrations of O2 and NO3- and small concentrations of NH4+ and dissolved organic carbon in the aquifer indicate that N2O in the central High Plains aquifer was produced primarily by nitrification. Calculations indicate that the flux of N2O from the central High Plains aquifer to the atmosphere from well pumping and groundwater discharge to streams was not a significant source of atmospheric N2O.Nitrogen-enriched groundwater has been proposed as an important anthropogenic source of atmospheric nitrous oxide (N2O), yet few measurements of N2O in large aquifer systems have been made. Concentrations of N2O in water samples collected from the 124000 km2 central High Plains aquifer in 1999 ranged from < 1 to 940 nM, with a median concentration of 29 nM (n = 123). Eighty percent of the N2O concentrations exceeded the aqueous concentration expected from equilibration with atmospheric N2O. Measurements of N2O, NO3-, and 3H in unsaturated-zone sediments, recently recharged groundwater, and older groundwater indicate that concentrations of N2O in groundwater increased over time and will likely continue to increase in the future as N-enriched water recharges the aquifer. Large concentrations of O2 and NO3- and small concentrations of NH4+ and dissolved organic carbon in the aquifer indicate that N2O in the central High Plains aquifer was produced primarily by nitrification. Calculations indicate that the flux of N2O from the central High Plains aquifer to the atmosphere from well pumping and groundwater discharge to streams was not a significant source of atmospheric N2O.Water samples were collected from 92 domestic wells, 16 monitoring wells and 15 public-supply wells in the High Plains Aquifer in 1999, and concentrations of nitrous oxide were measured. The groundwater concentrations ranged from less than 1 to 940 nM. Concentrations expressed as a percent of saturation in water ranged from less than 10 to 9690%. A significant decrease was noted in N2O concentrations with increasing depth of the well screen below the water table, and a significant positive correlation was found between the concentrations of N2O and nitrate. The small area-averaged N2O emission rate for the aquifer indicated that it was not an important component of the atmospheric N2O budget, but the importance could increase as groundwater N2O concentrations increase.

Environmental Science & Technology↗
USGS1998

N2O emissions from a nitrogen-enriched river

Nitrous oxide (N2O) emissions from the South Platte River in Colorado were measured using closed chambers in the fall, winter, and summer of 1994- 1995. The South Platte River was enriched in inorganic N (9-800 ??M) derived from municipal wastewater effluent and groundwater return flows from irrigated agricultural fields. River water was as much as 2500% supersaturated with N2O, and median N2O emission rates from the river surface ranged from less than 90 to 32 600 ??g-N m-2 d-1. Seventy-nine percent of the variance in N2O emission rates was explained by concentrations of total inorganic N in river water and by water temperature. The estimated total annual N2O emissions from the South Platte River were 2 x 1013-6 x 1013 ??g-N yr-1. This amount of annual N2O emissions was similar to the estimated annual N2O emissions from all primary municipal wastewater treatment processes in the United States (1). Results from this study indicate that N-enriched rivers could be important anthropogenic sources of N2O to the atmosphere. However, N2O emission measurements from other N-enriched rivers are needed to better quantify this source.Nitrous oxide (N2O) emissions from the South Platte River in Colorado were measured using closed chambers in the fall, winter, and summer of 1994-1995. The South Platte River was enriched in inorganic N (9-800 ??M) derived from municipal wastewater effluent and groundwater return flows from irrigated agricultural fields. River water was as much as 2500% supersaturated with N2O, and median N2O emission rates from the river surface ranged from less than 90 to 32 600 ??g-N m-2 d-1. Seventy-nine percent of the variance in N2O emission rates was explained by concentrations of total inorganic N in river water and by water temperature. The estimated total annual N2O emissions from the South Platte River were 2??1013-6??1013 ??g-N yr-1. This amount of annual N2O emissions was similar to the estimated annual N2O emissions from all primary municipal wastewater treatment processes in the United States. Results from this study indicate that N-enriched rivers could be important anthropogenic sources of N2O to the atmosphere. However, N2O emission measurements from other N-enriched rivers are needed to better quantify this source.

Environmental Science & Technology↗
USGS1998

Nitrous oxide fluxes from a claypan soil overlying nitrate-enriched glacial drift

The closed chamber method was used to assess nitrous oxide (N2O) fluxes from corn (Zea mays, L.) fields during the 1995 growing season. The study area was characterized by a claypan soil overlying a nitrate (NO31-)-enriched glacial-drift aquifer. Denitrification produced N2O fluxes of 0.2-6.9 g ha-1 hr-1 early in the growing season. Fluxes increased with increasing soil temperature, soil water potential, and soil saturation. However, greatly diminished N2O fluxes (0.001-0.09 gha-1 hr-1) occurred when soil saturation increased to 94 percent. Losses of N2O increased linearly during the day and decreased at night, probably because of declining soil temperatures. Declines in soil saturation (less than 80 percent) and soil moisture potential (less than -10 kPa) produced late season N2O fluxes (0.03-0.8 g ha-1 hr-1) attributable to nitrification. Results indicate that denitrification would not significantly reduce claypan soil NO31- concentrations.

Journal of Environmental Hydrology↗
USGS1997

Effect of nitrate, organic carbon, and temperature on potential denitrification rates in nitrate-rich riverbed sediments

A study conducted in 1994 as part of the US Geological Survey's National Water-Quality Assessment Program, South Platte River Basin investigation, examined the effect of certain environmental factors on potential denitrification rates in nitrate-rich riverbed sediments. The acetylene block technique was used to measure nitrous oxide (N2O) production rates in laboratory incubations of riverbed sediments to evaluate the effect of varying nitrate concentrations, organic carbon concentrations and type, and water temperature on potential denitrification rates. Sediment incubations amended with nitrate, at concentrations ranging from 357 to 2142 ??mol l-1 (as measured in the field), produced no significant increase (P > 0.05) in N2O production rates, indicating that the denitrification potential in these sediments was not nitrate limited. In contrast, incubations amended with acetate as a source of organic carbon, at concentrations ranging from 0 to 624 ??mol l-1, produced significant increases (P < 0.05) in N2O production rates with increased organic carbon concentration, indicating that the denitrification potential in these sediments was organic carbon limited. Furthermore, N2O production rates also were affected by the type of organic carbon available as an electron donor. Acetate and surface-water-derived fulvic acid supported higher N2O production rates than groundwater-derived fulvic acid or sedimentary organic carbon. Lowering incubation temperatures from 22 to 4??C resulted in about a 77% decrease in the N2O production rates. These results help to explain findings from previous studies indicating that only 15-30% of nitrate in groundwater was denitrified before discharging to the South Platte River and that nitrate concentrations in the river generally were higher in winter than in summer.

Journal of Hydrology↗
USGS1997

Distribution of terminal electron-accepting processes in an aquifer having multiple contaminant sources

Concentrations of electron acceptors, electron donors, and H2 in groundwater were measured to determine the distribution of terminal electron-accepting processes (TEAPs) in an alluvial aquifer having multiple contaminant sources. Upgradient contaminant sources included two separate hydrocarbon point sources, one of which contained the fuel oxygenate methyl tertbutyl ether (MTBE). Infiltrating river water was a source of dissolved NO31 SO4 and organic carbon (DOC) to the downgradient part of the aquifer. Groundwater downgradient from the MTBE source had larger concentrations of electron acceptors (dissolved O2 and SO4) and smaller concentrations of TEAP end products (dissolved inorganic C, Fe2+ and CH4) than groundwater downgradient from the other hydrocarbon source, suggesting that MTBE was not as suitable for supporting TEAPs as the other hydrocarbons. Measurements of dissolved H2 indicated that SO4 reduction predominated in the aquifer during a period of high water levels in the aquifer and river. The predominant TEAP shifted to Fe3+ reduction in upgradient areas after water levels receded but remained SO4 reducing downgradient near the river. This distribution of TEAPs is the opposite of what is commonly observed in aquifers having a single contaminant point source and probably reflects the input of Dec and SO4 to the aquifer from the river. Results of this study indicate that the distribution of TEAPs in aquifers having multiple contaminant sources depends on the composition and location of the contaminants and on the availability of electron acceptors.

Applied Geochemistry↗
USGS1996

Shallow ground-water quality beneath a major urban center: Denver, Colorado, USA

A survey of the chemical quality of ground water in the unconsolidated alluvial aquifer beneath a major urban center (Denver, Colorado, USA) was performed in 1993 with the objective of characterizing the quality of shallow ground-water in the urban area and relating water quality to land use. Thirty randomly selected alluvial wells were each sampled once for a broad range of dissolved constituents. The urban land use at each well site was sub- classified into one of three land-use settings: residential, commercial, and industrial. Shallow ground-water quality was highly variable in the urban area and the variability could be related to these land-use setting classifications. Sulfate (SO4) was the predominant anion in most samples from the residential and commercial land-use settings, whereas bicarbonate (HCO3) was the predominant anion in samples from the industrial land-use setting, indicating a possible shift in redox conditions associated with land use. Only three of 30 samples had nitrate concentrations that exceeded the US national drinking-water standard of 10 mg l-1 as nitrogen, indicating that nitrate contamination of shallow ground water may not be a serious problem in this urban area. However, the highest median nitrate concentration (4.2 mg l-1) was in samples from the residential setting, where fertilizer application is assumed to be most intense. Twenty-seven of 30 samples had detectable pesticides and nine of 82 analyzed pesticide compounds were detected at low concentrations, indicating that pesticides are widely distributed in shallow ground water in this urban area. Although the highest median total pesticide concentration (0.17 ??g l-1) was in the commercial setting, the herbicides prometon and atrazine were found in each land-use setting. Similarly, 25 of 29 samples analyzed had detectable volatile organic compounds (VOCs) indicating these compounds are also widely distributed in this urban area. The total VOC concentrations in sampled wells ranged from nondetectable to 23 442 ??g l-1. Widespread detections and occasionally high concentrations point to VOCs as the major anthropogenic ground-water impact in this urban environment. Generally, the highest VOC concentrations occurred in samples from the industrial setting. The most frequently detected VOC was the gasoline additive methyl tertbutyl ether (MTBE, in 23 of 29 wells). Results from this study indicate that the quality of shallow ground water in major urban areas can be related to land-use settings. Moreover, some VOCs and pesticides may be widely distributed at low concentrations in shallow ground water throughout major urban areas. As a result, the differentiation between point and non-point sources for these compounds in urban areas may be difficult.

Journal of Hydrology↗
USGS1996

Denitrification and mixing in a stream-aquifer system: Effects on nitrate loading to surface water

Ground water in terrace deposits of the South Platte River alluvial aquifer near Greeley, Colorado, USA, had a median nitrate concentration of 1857 μmol l −1 . Median nitrate concentrations in ground water from adjacent floodplain deposits (468 μmol l −1 ) and riverbed sediments (461 μmol l −1 ), both of which are downgradient from the terrace deposits, were lower than the median concentration in the terrace deposits. The concentrations and δ 15 N values of nitrate and N 2 in ground water indicated that denitrifying activity in the floodplain deposits and riverbed sediments accounted for 15–30% of the difference in nitrate concentrations. Concentrations of Cl − and SiO 2 indicated that mixing between river water and ground water in the floodplain deposits and riverbed sediments accounted for the remainder of the difference in nitrate concentrations. River flux measurements indicated that ground-water discharge in a 7.5 km segment of river had a nitrate load of 1718 kg N day − and accounted for about 18% of the total nitrate load in the river at the downstream end of that segment. This nitrate load was 70% less than the load predicted on the basis of the median nitrate concentration in the terrace deposits and assuming no denitrification or mixing in the aquifer. Water exchange between the river and aquifer caused ground water that originally discharged to the river to reenter denitrifying sediments in the riverbed and floodplain, thereby further decreasing the nitrate load in this stream—aquifer system. Results from this study indicated that denitrification and mixing within akluvial aquifer sediments may substantially decrease the nitrate load added to rivers by discharging ground water.

Journal of Hydrology↗
USGS1995

Water-quality assessment of the South Platte River basin, Colorado, Nebraska, and Wyoming: Analysis of available nutrient, suspended-sediment, and pesticide data, water years 1980-92

Nutrient, suspended-sediment, and pesticide data from surface- and ground-water sites in the South Platte River Basin for water years 1980-92 were compiled, screened, and interpreted. This activity is part of the U.S. Geological Survey's National Water Quality Assessment (NAWQA) program. The analysis of existing water-quality data provides a perspective on recent water-quality conditions in the South Platte River Basin, evaluations of the strengths and weaknesses of available data, and implications for water-quality issues and future study priorities and design. Most data analyzed were collected by the U.S. Geological Survey. Additionally, data from three local agencies were used in the analysis. A total of 3,484 samples from 54 surface-water sites and 107 wells were used in the analysis. The areal distribution of nutrient samples collected from surface-water and ground-water sites were sufficient in number and areal distribution to describe current water- quality conditions throughout the basin, but data were not sufficient to analyze factors and processes affecting water quality. However, suspended- sediment and pesticide data were sparse in their distribution with respect to time, space, and flow regime, and were sufficient only to provide a preliminary description of conditions in the basin.

Colorado, Nebraska, Wyoming↗