Geology topics

Robert J. Gilliom

Publications and source records attributed to Robert J. Gilliom.

At least 55 records · Page 3Linked to original sources

USGS2006

Development and application of a regression equation for estimating the occurrence of atrazine in shallow ground water beneath agricultural areas of the United States

Results from 52 ground-water studies throughout the United States were used to examine relations between the occurrence of atrazine in shallow ground water in agricultural settings and explanatory variables that describe the natural setting, agricultural-management practices, and the type and amount of development in each area. The explanatory variables that were found to be correlated with atrazine occurrence were soil-infiltration rates, presence of artificial drainage (tile drains or trenches), available water-holding capacity of soils, soil permeability, amount of study area using ground water for irrigation source (as percentage of total area), amount of agricultural land (as percentage of total area), and intensity of atrazine use. Ordinary least-squares regression equations that used one or more of these explanatory variables describe as much as 58 percent of the variation in atrazine-detection frequencies. Application of a multivariate equation to unmonitored agricultural areas across the conterminous United States illustrates that atrazine use alone is insufficient for estimating the occurrence of atrazine in shallow ground water. Instead, areas in which soil characteristics and agricultural-management practices favor the movement of water from land surface to the water table and that also have intensive atrazine use are the most vulnerable to atrazine contamination.

Scientific Investigations Report↗

USGS2006

Pesticide toxicity index for freshwater aquatic organisms, 2nd edition

The U.S. Geological Survey's National Water-Quality Assessment (NAWQA) Program is designed to assess current water-quality conditions, changes in water quality over time, and the effects of natural and human factors on water quality for the Nation's streams and ground-water resources. For streams, one of the most difficult parts of the assessment is to link chemical conditions to effects on aquatic biota, particularly for pesticides, which tend to occur in streams as complex mixtures with strong seasonal patterns. A Pesticide Toxicity Index (PTI) was developed that combines pesticide exposure of aquatic biota (measured concentrations of pesticides in stream water) with acute toxicity estimates (standard endpoints from laboratory bioassays) to produce a single index value for a sample or site. The development of the PTI was limited to pesticide compounds routinely measured in NAWQA studies and to toxicity data readily available from existing databases. Qualifying toxicity data were found for one or more types of test organisms for 124 of the 185 pesticide compounds measured in NAWQA samples, but with a wide range of available bioassays per compound (1 to 232). In the databases examined, there were a total of 3,669 bioassays for the 124 compounds, including 398 48-hour EC50 values (concentration at which 50 percent of test organisms exhibit a sublethal response) for freshwater cladocerans, 699 96-hour LC50 values (concentration lethal to 50 percent of test organisms) for freshwater benthic invertebrates, and 2,572 96-hour LC50 values for freshwater fish. The PTI for a particular sample is the sum of toxicity quotients (measured concentration divided by the median toxicity concentration from bioassays) for each detected pesticide, and thus, is based on the concentration addition model of pesticide toxicity. The PTI can be calculated for specific groups of pesticides and for specific taxonomic groups. Although the PTI does not determine whether water in a sample is toxic to aquatic organisms, its values can be used to rank or compare the toxicity of samples or sites on a relative basis for use in further analysis or additional assessments. The PTI approach may be useful as a basis for comparing the potential significance of pesticides in different streams on a common basis, for evaluating relations between pesticide exposure and observed biological conditions, and for prioritizing where further studies are most needed.

Scientific Investigations Report↗

USGS2005

Pesticides in ground water of the United States, 1992-1996

During the first cycle of the National Water Quality Assessment (1992–1996), ground water in 20 of the nation's major hydro-logic basins was analyzed for 90 pesticide compounds (pesticides and degradates). One or more of the pesticide compounds examined were detected at 48.4% of the 2485 ground water sites sampled. However, approximately 70% of the sites where pesticides were detected, two or more pesticide compounds analyzed were present–documenting the prevalence of pesticide mixtures in ground water. The pesticide concentrations encountered were generally low, with the median total concentration (summation of concentrations for the 90 pesticide compounds) being 0.046 μg/L. Pesticides were commonly detected in shallow ground water beneath both agricultural (60.4%) and urban (48.5%) areas. This is an important finding because, although agricultural activities have long been associated with pesticide contamination, urban areas have only recently been recognized as a potential source of these types of compounds. Pesticides with higher frequencies of detection were generally those with more extensive use, greater environmental persistence, and greater mobility in ground water (lower soil-water partition coefficients).

Ground Water↗

USGS2004

Development and Application of Watershed Regressions for Pesticides (WARP) for Estimating Atrazine Concentration Distributions in Streams

Regression models were developed for predicting atrazine concentration distributions in rivers and streams, using the Watershed Regressions for Pesticides (WARP) methodology. Separate regression equations were derived for each of nine percentiles of the annual distribution of atrazine concentrations and for the annual time-weighted mean atrazine concentration. In addition, seasonal models were developed for two specific periods of the year--the high season, when the highest atrazine concentrations are expected in streams, and the low season, when concentrations are expected to be low or undetectable. Various nationally available watershed parameters were used as explanatory variables, including atrazine use intensity, soil characteristics, hydrologic parameters, climate and weather variables, land use, and agricultural management practices. Concentration data from 112 river and stream stations sampled as part of the U.S. Geological Survey's National Water-Quality Assessment and National Stream Quality Accounting Network Programs were used for computing the concentration percentiles and mean concentrations used as the response variables in regression models. Tobit regression methods, using maximum likelihood estimation, were used for developing the models because some of the concentration values used for the response variables were censored (reported as less than a detection threshold). Data from 26 stations not used for model development were used for model validation. The annual models accounted for 62 to 77 percent of the variability in concentrations among the 112 model development stations. Atrazine use intensity (the amount of atrazine used in the watershed divided by watershed area) was the most important explanatory variable in all models, but additional watershed parameters significantly increased the amount of variability explained by the models. Predicted concentrations from all 10 models were within a factor of 10 of the observed concentrations at most model development and model validation stations. Results for the two sets of seasonal models were similar. Concentration distributions derived from the seasonal-model predictions provided additional information compared to distributions derived from the annual models.

Water-Resources Investigations Report↗

USGS2001

Pesticide toxicity index for freshwater aquatic organisms

The U.S. Geological Survey's National Water-Quality Assessment (NAWQA) Program is designed to assess current water-quality conditions, changes in water quality over time, and the effects of natural and human factors on water quality for the Nation's streams and ground-water resources. For streams, one of the most difficult parts of the assessment is to link chemical conditions to effects on aquatic biota, particularly for pesticides, which tend to occur in streams as complex mixtures with strong seasonal patterns. A Pesticide Toxicity Index (PTI) was developed that combines pesticide exposure of aquatic biota (measured concentrations of pesticides in stream water) with toxicity estimates (standard endpoints from laboratory bioassays) to produce a single index value for a sample or site. The development of the PTI was limited to pesticide compounds routinely measured in NAWQA studies and to toxicity data readily available from existing databases. Qualifying toxicity data were found for one or more types of test organisms for 75 of the 83 pesticide compounds measured in NAWQA samples, but with a wide range of bioassays per compound (1 to 65). There were a total of 2,824 bioassays for the 75 compounds, including 287 48-hour EC50 values (concentration at which 50 percent of test organisms exhibit a nonlethal response) for freshwater cladocerans, 585 96-hour LC50 values (concentration lethal to 50 percent of test organisms) for freshwater benthic invertebrates, and 1,952 96-hour LC50 values for freshwater fish. The PTI for a particular sample is the sum of toxicity quotients (measured concentration divided by the median toxicity concentration from bioassays) for each detected pesticide. The PTI can be calculated for specific groups of pesticides and for specific taxonomic groups.While the PTI does not determine whether water in a sample is toxic, its values can be used to rank or compare the toxicity of samples or sites on a relative basis for use in further analysis or additional assessments. The PTI approach may be useful as a basis for comparing the potential significance of pesticides in different streams on a common basis, for evaluating relations between pesticide exposure and observed biological conditions, and for prioritizing where further studies are most needed.

Water-Resources Investigations Report↗

USGS1999

Summary and evaluation of pesticides in field blanks collected for the National Water-Quality Assessment Program, 1992-95

Field blanks are quality-control samples used to assess contamination in environmental water samples. Contamination is the unintentional introduction of a chemical (pesticides in this instance) into an environmental water sample from sources such as inadequately cleaned equipment, dirty hands, dust, rain, or fumes. Contamination causes a positive bias in analytical measurements that may need to be considered in the analysis and interpretation of the environmental data. Estimates of pesticide contamination in environmental water samples collected for the National Water-Quality Assessment (NAWQA) Program are used to qualify, where needed, interpretations of the occurrence and distribution of pesticides in the surface and ground waters of the United States. Field blanks collected from 1992 to 1995 as part of the NAWAQA Program were analyzed for 88 pesticides and pesticide metabolites. Of 47 pesticides determined by gas chromatography/mass spectrometry, 23 were detected at least once in 175 surface-water field blanks and 15 were detected at least once in 145 ground-water field blanks. The most frequently detected pesticides in surface-water field blanks were atrazine (in 10.9 percent of blanks), simazine (9.1 percent), and metolachlor (4.6 percent). The most frequently detected pesticides in ground-water field blanks were p,p' -DDE (4.1 percent) and atrazine (2.8 percent). The maximum pesticide concentration detected by gas chromatography/mass spectrometry in a surface-water field blank was 0.120 microgram per liter (µg/L) for pronamide; the maximum concentration detected in a ground-water field blank was 0.013 µg /L for chlorpyrifos and prometon. Of 41 pesticides determined by high-performance liquid chromatography, diuron and 2,4-D were detected once in 109 surface-water field blanks and bromacil, diuron, and fenuron were detected once in 104 ground-water field blanks. Except for a detection of 2,4-D at 0.230 µg /L, the detectable concentrations of these pesticides were less than or equal to 0.020 µg /L. Field blanks showed no evidence of contamination by most pesticides. Of the 88 pesticides for which the field blanks were analyzed, 63 were not detected in field blanks from surface-water sites and 70 were not detected in field blanks from ground-water sites. Therefore, environmental data for the pesticides not detected in field blanks can be interpreted without qualification for contamination. Field blanks did show evidence of contamination by some pesticides. Most of the pesticides detected in field blanks, however, were detected more frequently and at higher concentrations in environmental water samples. Two criteria were used to evaluate the need to consider contamination in water-quality assessments: (1) a ratio of the frequency of pesticide detection in environmental water samples to the frequency of detection in field blanks of 5.0 or less and (2) a ratio of the median concentration detected in environmental water samples to the maximum concentration detected in field blanks of 2.0 or less. These criteria indicate that contamination, for the majority of the pesticide data collected for the NAWQA Program, probably does not need to be considered in the analysis and interpretation of (1) the frequency of pesticide detection or (2) the median concentration of pesticides detected. Contamination must be considered, however, in detection frequency for cispermethrin, pronamide, p,p' -DDE, pebulate, propargite, ethalfluralin, and triallate in surface water and fenuron, benfluralin, pronamide, cis-permethrin, triallate, chlorpyrifos, trifluralin, propanil, p,p' -DDE, bromacil, dacthal, diazinon, and diuron in ground water. Contamination also must be considered in median concentrations detected for pronamide, p,p' -DDE, propargite, napropamide, and triallate in surface water and benfluralin, cis-permethrin, triallate, chlorpyrifos, trifluralin, p,p' -DDE, dacthal, and diazinon in ground water.

Open-File Report↗

USGS1999

Pesticides in streams of the United States : initial results from the National Water-Quality Assessment Program

Water samples from 58 rivers and streams across the United States were analyzed for pesticides as part of the National Water-Quality Assessment Program of the U.S. Geological Survey. The sampling sites represent 37 diverse agricultural basins, 11 urban basins, and 10 basins with mixed land use. Forty-six pesticides and pesticide degradation products were analyzed in approximately 2,200 samples collected from 1992 to 1995. The target compounds account for approximately 70 percent of national agricultural use in terms of the mass of pesticides applied annually. All the target compounds were detected in one or more samples. Herbicides generally were detected more frequently and at higher concentrations than insecticides. Nationally, 11 herbicides, 1 herbicide degradation product, and 3 insecticides were detected in more than 10 percent of samples. The number of target compounds detected at each site ranged from 7 to 37. The herbicides atrazine, metolachlor, prometon, and simazine were detected most frequently; among the insecticides, carbaryl, chlorpyrifos, and diazinon were detected the most frequently. Distinct differences in pesticide occurrence were observed in streams draining the various agricultural settings. Relatively high levels of several herbicides occurred as seasonal pulses in corn-growing areas. Several insecticides were frequently detected in areas where the dominant crops consist of orchards and vegetables. The number of pesticides detected and their concentrations were lower in wheat-growing areas than in most other agricultural areas. In most urban areas, the herbicides prometon and simazine and the insecticides carbaryl, chlorpyrifos, diazinon, and malathion were commonly detected. Concentrations of pesticides rarely exceeded standards and criteria established for drinking water, but some pesticides commonly exceeded criteria established for the protection of aquatic life.

Water-Resources Investigations Report↗

USGS1998

Occurrence of pesticides in shallow groundwater of the United States: initial results from the National Water-Quality Assessment program

The first phase of intensive data collection for the National Water-Quality Assessment (NAWQA) was completed during 1993−1995 in 20 major hydrologic basins of the United States. Groundwater land-use studies, designed to sample recently recharged groundwater (generally within 10 years) beneath specific land-use and hydrogeologic settings, are a major component of the groundwater quality as sessment for NAWQA. Pesticide results from the 41 land-use studies conducted during 1993−1995 indicate that pesticides were commonly detected in shallow groundwater, having been found at 54.4% of the 1034 sites sampled in agricultural and urban settings across the United States. Pesticide concentrations were generally low, with over 95% of the detections at concentrations less than 1 μg/L. Of the 46 pesticide compounds examined, 39 were detected. The compounds detected most frequently were atrazine (38.2%), deethylatrazine (34.2%), simazine (18.0%), metolachlor (14.6%), and prometon (13.9%). Statistically significant relations were observed between frequencies of detection and the use, mobility, and persistence of these compounds. Pesticides were commonly detected in both agricultural (56.4%; 813 sites) and urban (46.6%; 221 sites) settings. Frequent detections of pesticides in urban areas indicate that, as is the case with agricultural pesticide use in agricultural areas, urban and suburban pesticide use significantly contribute to pesticide occurrence in shallow groundwater. Although pesticides were detected in groundwater sampled in urban areas and all nine of the agricultural land-use categories examined, significant variations in occurrence were observed among these categories. Maximum contaminant levels (MCLs) established by the U.S. Environmental Protection Agency for drinking water were exceeded for only one pesticide (atrazine, 3 μg/L) at a single location. However, MCLs have been established for only 25 of the 46 pesticide compounds examined, do not cover pesticide degradates, and, at present, do not take into account additive or synergistic effects of combinations of pesticide compounds or potential effects on nearby aquatic ecosystems.

Environmental Science & Technology↗

USGS1998

Methods for comparing water-quality conditions among National Water-Quality Assessment Study Units, 1992-1995

The National Water-Quality Assessment is based on intensive investigations of stream and ground-water quality in selected major hydrologic basins (study units) of the United States. One objective of the national assessment is to comparatively evaluate water-quality conditions within and among the different study units. Methods were developed to compare the water-quality conditions of 20 study units that were studied during 1992-1995. Two approaches were taken: (1) water-quality conditions for each study unit were ranked in relation to the findings for all study units, and (2) water-quality conditions for each study unit were compared to established criteria for the protection of human health and aquatic life. Separate rankings were developed for several major characteristics of water quality by using selected combinations of measured values for individual constituents or properties. The water-quality characteristics that were evaluated for streams were nutrients and pesticides in water, organochlorine pesticides and polychlorinated biphenyls in bed sediment and tissue, semivolatile organic compounds and trace elements in bed sediment, fish community degradation, and stream habitat degradation. The water-quality characteristics that were evaluated for ground water were nitrate, pesticides, volatile organic compounds, dissolved solids, and radon. The water-quality rankings are relative strictly to the distribution of conditions measured at sampling sites included in developing the method. Sites in the first 20 National Water-Quality Assessment study units include a broad range of environmental settings, but are not a statistically representative sample of the Nation. > To supplement the relative rankings, established water-quality criteria were used to indicate where particular constituents may have adverse effects, and thus merit further investigation. Established water-quality criteria, which provide consistent benchmarks for national comparisons of individual constituents, were selected from a variety of sources and applied to specific constituents in the specific medium (water or sediment) appropriate for each criterion.

Open-File Report↗

USGS1997

Classification and Mapping of Agricultural Land for National Water-Quality Assessment

Agricultural land use is one of the most important influences on water quality at national and regional scales. Although there is great diversity in the character of agricultural land, variations follow regional patterns that are influenced by environmental setting and economics. These regional patterns can be characterized by the distribution of crops. A new approach to classifying and mapping agricultural land use for national water-quality assessment was developed by combining information on general land-use distribution with information on crop patterns from agricultural census data. Separate classification systems were developed for row crops and for orchards, vineyards, and nurseries. These two general categories of agricultural land are distinguished from each other in the land-use classification system used in the U.S. Geological Survey national Land Use and Land Cover database. Classification of cropland was based on the areal extent of crops harvested. The acreage of each crop in each county was divided by total row-crop area or total orchard, vineyard, and nursery area, as appropriate, thus normalizing the crop data and making the classification independent of total cropland area. The classification system was developed using simple percentage criteria to define combinations of 1 to 3 crops that account for 50 percent or more or harvested acreage in a county. The classification system consists of 21 level I categories and 46 level II subcategories for row crops, and 26 level I categories and 19 level II subcategories for orchards, vineyards, and nurseries. All counties in the United States with reported harvested acreage are classified in these categories. The distribution of agricultural land within each county, however, must be evaluated on the basis of general land-use data. This can be done at the national scale using 'Major Land Uses of the United States,' at the regional scale using data from the national Land Use and Land Cover database, or at smaller scales using locally available data.

Circular↗

USGS1997

Reconnaissance of 17 beta-estradiol, 11-ketotestosterone, vitellogenin, and gonad histopathology in common carp of United States streams; potential for contaminant-induced endocrine disruption

A reconnaissance of sex steroid hormones and other biomarkers in common carp was used to assess whether endocrine disruption may be occurring in fish in United States streams, to evaluate relations between endocrine disruption and contaminant levels, and to determine requirements for further studies. 17?-estradiol, 11-ketotestosterone, vitellogenin, and gonadal histopathology were measured in adult carp (usually 10--15 for each sex) at 25 sites (647 fish), representing a wide range of environmental settings typical of major regions of the nation. Fish were collected during August--December 1994, a period of gonadal maturation after spawning. Contaminants evaluated were organochlorine pesticides and polychlorinated biphenyls in tissue; phthalates, phenols, and polycyclic aromatic hydrocarbons in bed sediment; and dissolved pesticides in water. Mean site concentrations of steroid hormones spanned two orders of magnitude for both sexes. No significant regional differences in steroid hormones were detected for males, but females from the Northern and Southern Midcontinent were significantly different from other regions of the country in one or both hormones. Within all regions there were significant differences between sites in one or both hormones for both sexes. Most correlation coefficients between biomarkers and contaminants were negative. Contaminants that had significant (a=0.05) correlations with biomarkers were organochlorine pesticides, phenols, and dissolved pesticides. The strongest pattern common to both males and females was a negative correlation between the hormone ratio (E2/11-KT) and dissolved pesticides. The significant site-to-site differences in biomarkers, and the presence of significant correlations between biomarkers and contaminants, are evidence that fish in some streams may be experiencing endocrine disruption. Improved information is needed to evaluate whether endocrine disruption is actually occurring and if there are reproductive effects on individual or populations of carp or other species. Future studies should shift to more intensive study of fewer sites, including reference and contaminated sites, in order to address these additional questions.

Open-File Report↗

USGS1995

Design of the National Water-Quality Assessment Program; occurrence and distribution of water-quality conditions

The National Water-Quality Assessment Program assesses the status of and trends in the quality of the Nation's ground- and surface-water resources. The occurrence and distribution assessment component characterizes broad-scale water-quality conditions in relation to major contaminant sources and background conditions in each study area. The surface-water design focuses on streams. The ground-water design focuses on major aquifers, with emphasis on recently recharged ground water associated with human activities.

Circular↗

USGS1985

Pesticides in the nation's rivers, 1975-1980, and implications for future monitoring

Water samples were taken four times per year and bed-sediment samples two times per year during 1975-80 at 160 to 180 stations on major rivers of the United States. Samples were analyzed for 18 insecticides and 4 herbicides, which together accounted for about one-third of the total amount of all pesticides applied to major crops during 1975-80. Fewer than 10 percent of almost 3,000 water samples and fewer than 20 percent of almost 1,000 bed-sediment samples contained reportable concentrations of any of the compounds. The patterns of detection result from a combination of widely variable detection capabilities, chemical properties, and use. Most detections in water samples were of relatively persistent yet soluble compounds: atrazine (4.8 percent of samples), diazinon (1.2), and lindane (1.1). Most detections in bed-sediment samples were of the hydrophobic and persistent insecticides: DDE (17 percent of samples), DDD (12), dieldrin (12), chlordane (9.9), and DDT (8.5). Only for atrazine in water, and for DDE, DDD, DDT, and chlordane in bed sediments, were geographic patterns of detection correlated (pH<0.10) with use on farms. Detections of organochlorine insecticides in both water and bed sediments appear to have erratically but gradually decreased during 1975-80. For the 1975-79 period, more stations had downtrends than had uptrends in bed-sediment levels of organochlorines. No clear trends were evident in concentrations of organophosphate insecticides or herbicides in either water or bed sediments. Findings suggest that future pesticide monitoring efforts must be responsive to changes in pesticides used and to geographic patterns of use. Different types of monitoring approaches are necesssary for chemicals having different chemical and physical properties. Before an effective dynamic monitoring effort can be designed, however, selected case studies are needed to characterize and refine sampling and analytical capabilities for different types of chemicals, river environments, and sample types.

Water Supply Paper↗

USGS1983

Estimation of nonpoint sources of phosphorus for lakes in the Puget Sound region, Washington

Control of eutrophication of lakes in watersheds undergoing development is facilitated by estimates of the amounts of phosphorus (P) that reach the lakes from areas under various types of land use. Using a mass-balance model, the writer calculated P loadings from present-day P concentrations measured in lake water and from other easily measured physical characteristics in a total of 28 lakes in drainage basins that contain only forest and residential land. The loadings from background sources (forest-land drainage and bulk precipitation) to each of the lakes were estimated by methods developed in a previous study. Differences between estimated present-day P loadings and loadings from background sources were attributed to changes in land use. The mean increase in annual P yield resulting from conversion of forest to residential land use was 7 kilograms per square kilometer, not including septic-tank system contributions. Calculated loadings from septic systems were found to correlate best with the number of near-shore dwellings around each lake in 1940. The regression equation expressing this relationship explained 36 percent of the sample variance. There was no significant correlation between estimated septic-tank system P loadings and numbers of dwellings present in 1960 or 1970. The evidence indicates that older systems might contribute more phosphorus to lakes than newer systems, and that there may be substantial time lags between septic-system installation and significant impacts on lake-water P concentrations. For lakes in basins that contain agricultural land, the P loading attributable to agriculture can be calculated as the difference between the estimated total loading and the sum of estimated loadings from all other (nonagricultural) sources. A comprehensive system for evaluating errors in all loading estimates is presented. The empirical relationships developed allow preliminary approximations of the cumulative impact that development has had on P loading and the amounts of P loading from generalized land-use categories for Puget Sound lowland lakes. In addition, the sensitivity of a lake to increased loading can be evaluated using the mass-balance model. The methods use data that are presently available for most lakes. All the estimates are most suitable for use in developing water-quality goals, setting priorities for lake studies, and designing studies of individual lakes. The suitability of the method for management of individual lakes will often be limited by relatively high levels of uncertainty, especially if the method is used to evaluate relatively small increases in P loading.

Washington↗

USGS1983

Relationships between water quality and phosphorus concentrations for lakes of the Puget Sound region, Washington

Predictive regression relationships between concentrations of total phosphorus (P) in near-surface waters (1m) and water-quality characteristics were developed for lakes in the Puget Sound region. Nitrogen-to-phosphorus ratios support the strategy of emphasizing total P for lake eutrophication assessment within the range of lake conditions represented. Relationships based on summer total P concentrations, rather than winter-spring total P, led to more successful prediction of mean summertime chlorophyll a concentrations and Secchi-disc transparencies. The 78 lakes evaluated in the study had an average mean concentration of total phosphorus in near-surface waters of 22 ?g/L during the summer with mean concentrations for individual lakes ranging from 7 to 76 ?g/L. For regression relationships based on summer total phosphorus concentrations, the standard error of predicted Secchi-disc transparencies is ?0.9 meters for phosphorus concentrations of 15 ?g/L or greater, and the standard error of predicted chlorophyll a concentrations is ?5 ?g/L. Predictions for the range of conditions evaluated favorably agreed with predictions from similar relationships developed by other researchers for different lakes. To supplement estimates of transparency and chlorophyll a from regression relationships, a broader system of water-quality groups, including recreational suitability and fish habitat quality, was developed from published information and our experience in the region.

Open-File Report↗

USGS1983

Estimation of nonpoint source loadings of phosphorus for lakes in the Puget Sound region, Washington

Control of eutrophication of lakes in watersheds undergoing development is facilitated by estimates of the amounts of phosphorus (P) that reach the lakes from areas under various types of land use. Using a mass-balance model, the author calculated P loadings from present-day P concentrations measured in lake water and from other easily measured physical characteristics in a total of 28 lakes in drainage basins that contain only forest and residential land. The loadings from background sources (forest-land drainage and bulk precipitation) to each of the lakes were estimated by methods developed in a previous study. Differences between estimated present-day P loadings and loadings from background sources were attributed to changes in land use. The mean increase in annual P yield resulting from conversion of forest to residential land use was 7 kilograms per square kilometer, not including septic tank system contributions. Calculated loadings from septic systems were found to correlate best with the number of near-shore dwellings around each lake in 1940. The regression equation expressing this relationship explained 36 percent of the sample variance. There was no significant correlation between estimated septic tank system P loadings and number of dwellings present in 1960 or 1970. The evidence indicates that older systems might contribute more phosphorus to lakes than newer systems, and that there may be substantial time lags between septic system installation and significant impacts on lake-water P concentrations. For lakes in basins that contain agricultural land, the P loading attributable to agriculture can be calculated as the difference between the estimated total loading and the sum of estimated loadings from nonagricultural sources. A comprehensive system for evaluating errors in all loading estimates is presented. The empirical relationships developed allow preliminary approximations of the cumulative impact development has had on P loading and the amounts of P loading from generalized land-use categories for Puget Sound lowland lakes. In addition, the sensitivity of a lake to increased loading can be evaluated using the mass-balance model. The data required are presently available for most lakes. Estimates of P loading are useful in developing water-quality goals, setting priorities for lake studies, and designing studies of individual lakes. The suitability of a method for management of individual lakes will often be limited by relatively high levels of uncertainty, especially if the method is used to evaluate relatively small increases in P loading.

Water Supply Paper↗

USGS1982

Lake phosphorus loading from septic systems by seasonally perched ground water, Puget Sound region, Washington

In a previous study, estimated phosphorus (P) loadings from septic systems to lakes in the Puget Sound region were found to be correlated with the presence of old homes around the lakes. In the present study, we assessed the movement of septic-effluent P in seasonally perched ground water near Pine Lake, a typical glacial-till lake in the region. This ground water occurs in soils overlying less permeable glacial till, which is prevalent around Pine Lake and many other lakes in the area. Water samples were taken from 15 shallow (<1.5 meters) wells installed 10-50 meters downgradient from seven septic systems 20 to 40 years old. The equivalent volumetric fraction of each sample consisting of undiluted effluent was estimated from chloride concentration. Using Monte Carlo analysis to account for the various sources of uncertainty, we found that, though movement of diluted septic effluent to the lake was common, transport of more than 1% of effluent P through the soil was probable (p > or = 0.5) for only 4 of 26 samples, was transport of more than 10% of effluent P probable. The highest probabilities of P movement were associated with two samples from a well that was downgradient from a drainfield located at the base of a hillslope depression where perched ground water concentrates and remains for extended periods. All evidence considered, most P loading to Pine Lake from septic systems appears to come from only a few older systems located in areas where perched ground-water flow and associated saturated soil conditions predominate for extended periods during the winter season. (USGS)

Open-File Report↗

USGS1980

Estimation of background loadings and concentrations of phosphorus for lakes in the Puget Sound region, Washington

For lakes in watersheds that include developed land, evaluation of eutrophication is facilitated by knowledge of changes in the lakes ' phosphorus concentration since development. A method is described for estimating background phosphorus concentration in lakes of the Puget Sound lowland, Washington. Using a mass-balance ' Vollenweider-type ' model, phosphorus loadings were calculated from present-day phosphorus concentrations measured in lake water and from easily measured physical characteristics for 24 lakes in undeveloped, or insignificantly developed, watersheds. Phosphorus loading from forest (undeveloped) land was derived for each lake as the difference between the calculated phosphorus loading directly to the lake 's surface and loading by bulk precipitation (estimated from other studies). Forest-land loading to each lake was converted to the yield (mass per unit area) of the forested part of the watershed. The phosphorus yield from forest land was related to annual runoff, and the regression equation expressing this relationship explained 73% of the sample variance. By applying that regression equation to the appropriate annual-runoff data, the yield of phosphorus from forest land can be estimated for any lake in the study area. Phosphorus loading from forested land then can be added to direct loading by bulk precipitation to estimate background phosphorus loading for each lake. By applying the mass-balance model to calculated background loadings, background total-phosphorus concentration can also be calculated for all lakes in the study area that have stable thermal stratification during the summer. The standard error of estimate for calculated background loadings and concentrations averages about 25%. (USGS)

Open-File Report↗