Watershed characteristics and water-quality trends and loads in 12 watersheds in Gwinnett County, Georgia

The U.S. Geological Survey, in cooperation with Gwinnett County Department of Water Resources, established a Long-Term Trend Monitoring (LTTM) program in 1996. The LTTM program is a comprehensive, long-term, water-quantity and water-quality monitoring program designed to document and analyze the hydrologic and water-quality conditions of selected watersheds of Gwinnett County, Georgia. Waterquality monitoring initially began in six watersheds and was expanded to another six watersheds in 2001. As part of the LTTM program, streamflow, precipitation, water temperature, specific conductance, and turbidity were measured continuously at the 12 watershed monitoring stations for water years1 2004–09. In addition, discrete waterquality samples were collected seasonally from May through October (summer) and November through April (winter), including one base-flow and three stormflow event composite samples, during the study period. Samples were analyzed for nutrients (nitrogen and phosphorus), total organic carbon, trace elements (total lead and total zinc), total dissolved solids, and total suspended sediment (total suspended solids and suspended-sediment concentrations). The sampling scheme was designed to identify variations in water quality both hydrologically and seasonally. The 12 watersheds were characterized for basin slope, population density, land use for 2009, and the percentage of impervious area from 2000 to 2009. Precipitation in water years 2004–09 was about 18 percent below average, and the county experienced exceptional drought conditions and below average runoff in water years 2007 and 2008. Watershed water yields, the percentage of precipitation that results in runoff, typically are lower in low precipitation years and are higher for watersheds with the highest percentages of impervious areas. A comparison of base-flow and stormflow water-quality samples indicates that turbidity and concentrations of total ammonia plus organic nitrogen, total nitrogen, total phosphorus, total organic carbon, total lead, total zinc, total suspended 1A water year is the period from October 1 to September 30 and is designated by the year in which the period ends. solids, and suspended-sediment concentrations increased with increasing discharge at all watersheds. Specific conductance, however, decreased during stormflow at all watersheds, and total dissolved solids concentrations decreased during stormflow at a few of the watersheds. Total suspended solids and suspended-sediment concentrations typically were two orders of magnitude higher in stormflow samples, turbidities were about 1.5 orders of magnitude higher, total phosphorus and total zinc were about one order of magnitude higher, and total ammonia plus organic nitrogen, total nitrogen, total organic carbon, and total lead were about twofold higher than in base-flow samples. Seasonal patterns and long-term trends in flow-adjusted water-quality concentrations were identified for five representative constituents—total nitrogen, total phosphorus, total zinc, total dissolved solids, and total suspended solids. Seasonal patterns for all five constituents were fairly similar, with higher concentrations in the summer and lower concentrations in the winter. Significant linear long-term trends in stormflow composite concentrations were identified for 36 of the 60 constituent-watershed combinations (5 constituents multiplied by 12 watersheds) for the period of record through water year 2011. Significant trends typically were decreasing for total nitrogen, total phosphorus, total suspended solids, and total zinc and increasing for total dissolved solids. Total dissolved solids and total suspended solids trends had the largest magnitude changes per year. Stream water loads were estimated for 10 water-quality constituents. These estimates represent the cumulative effects of watershed characteristics, hydrologic processes, biogeochemical processes, climatic variability, and human influences on watershed water quality. Yields, in load per unit area, were used to compare loads from watersheds with different sizes. A load estimation approach developed for the Gwinnett County LTTM program that incorporates storm-event composited samples was used with some minor modifications. This approach employs the commonly used regression-model method. Concentrations were modeled as a function of discharge, time, season, and turbidity to improve model predictions and reduce errors in load estimates. Total suspended solids annual loads have been identified in Gwinnett County’s Watershed Protection Plan for target performance criterion. 2 Watershed Characteristics and Water-Quality Trends and Loads in 12 Watersheds in Gwinnett County, Georgia The amount of annual runoff is the primary factor in determining the amount of annual constituent loads. Below average runoff during water years 2004–09, especially during water years 2006–08, resulted in corresponding below average loads. Variations in constituent yields between watersheds appeared to be related to various watershed characteristics. Suspended sediment (total suspended solids and suspendedsediment concentrations) along with constituents transported predominately in solid phase (total phosphorus, total organic carbon, total lead, and total zinc) and total dissolved solids typically had higher yields from watersheds that had high percentages of impervious areas or high basin slope. High total nitrogen yields were also associated with watersheds with high percentages of impervious areas. Low total nitrogen, total suspended solids, total lead, and total zinc yields appear to be associated with watersheds that have a low percentage of high-density development. Total suspended solids yields were lower in drought years, water years 2007–08, from the combined effects of less runoff and the result of fewer, lower magnitude storms, which likely resulted in less surface erosion and lower stream sediment transport. Introduction Watershed surface-water quantity and water quality reflect and integrate the effects of watershed characteristics, inputs (for example, precipitation and dry deposition), hydrologic and biogeochemical processes, climatic variability (such as droughts and floods), and human influences. Changes in land use alter complex interactions that affect many processes within a watershed (MacDonald, 2000). Urbanization, with its associated increases in impervious area, can have a great effect on stream hydrology by (1) decreasing rainfall infiltration and groundwater recharge rates, resulting in lower stream base flow, and (2) increasing storm runoff, peak discharges, and flood flows (Leopold, 1968). Urbanization and the resulting hydrologic response can affect surface-water quality by increasing surface erosion, sediment transport, and pollutant loadings and by altering stream-channel stability. Best management practices (BMPs; for example, stormwater detention ponds, diversion structures, and stream buffers) and other erosion controls frequently are implemented to mitigate the effects of urbanization on stream hydrology and minimize sediment transport in urbanized areas. Monitoring specific water-quality constituents allows for the identification of trends and the computation of loads and yields to help attribute changes in water quality to natural variations as opposed to those related to changes in land use and watershed management strategies. The U.S. Geological Survey (USGS), in cooperation with Gwinnett County Department of Water Resources, established a comprehensive Long-Term Trend Monitoring (LTTM) program in 1996 to monitor, analyze, and quantify the magnitudes of pollutants and the effects of urbanization on six watersheds. In 2001, six additional watersheds were added to the LTTM program (fig. 1). These 12 watersheds were continuously monitored during water years (WYs) 2004–09 for water level (stage), discharge, precipitation, and water-quality properties (water temperature, specific conductance [SC], and turbidity). Three stormflow-composited samples and one base-flow sample were collected in both summer (May–October) and winter (November–April) of each year; samples were analyzed for nutrients, trace metals, total dissolved solids, and total suspended sediment (total suspended solids and suspended-sediment concentration). The sampling scheme was designed to identify variations in water quality both hydrologically and seasonally. The primary purpose of the monitoring program is to collect and interpret consistent, high-quality water-quantity and water-quality data. Watershed managers can use these data to make informed management decisions in order to maintain the designated uses of streams, protect aquatic habitats, and optimize the effectiveness of water-management practices (BMPs). Intensive, long-term monitoring of watershed characteristics, streamflow, and stream quality are essential to quantifying the impacts of specific land uses, point-source and nonpoint-source discharges, and management processes on surface water (water quantity, flow characteristics, and water quality). The LTTM program also fulfills requirements outlined in the Gwinnett County Watershed Protection Plan as well as requirements for the National Pollutant Discharge Elimination System (NPDES) permit and the Metropolitan North Georgia Water Planning District (MNGWPD). Purpose and Scope The purpose of this report is to summarize and analyze the hydrologic, water-quality, and land-use data collected as part of Gwinnett County’s LTTM program for 12 watersheds for the 6-year period WYs 2004–09. This report is an update to, and continuation of, the report by Landers and others (2007) that presented and analyzed data for the original six watersheds for WYs 1996–2003. The specific goals of this report are to: • Expand the watershed characterization reported in Landers and others (2007) to include the additional six watersheds added to the LTTM program; • Discu