ABSTRACT

Hydrologic studies conducted during 2003–2008 as part of the U.S. Geological Survey Cooperative Water Program with the City of Lawrenceville, Georgia, provide important data for the management of water resources. The Cooperative Water Program includes (1) hydrologic monitoring (precipitation, streamflow, and groundwater levels) to quantify baseline conditions in anticipation of expanded groundwater development, (2) surface-water-quality monitoring to provide an understanding of how stream quality is affected by natural (such as precipitation) and anthropogenic factors (such as impervious area), and (3) geologic studies to better understand groundwater flow and hydrologic processes in a crystalline rock setting.

The hydrologic monitoring network includes each of the two watersheds projected for groundwater development—the Redland–Pew Creek and upper Alcovy River watersheds—and the upper Apalachee River watershed, which serves as a background or control watershed because of its similar hydrologic and geologic characteristics to the other two watersheds. In each watershed, precipitation was generally greater during 2003–2005 than during 2006–2008, and correspondingly streamflow and groundwater levels decreased. In the upper Alcovy River and Redland–Pew Creek watersheds, groundwater level declines during 2003–2008 were mostly between 2 and 7 feet, with maximum observed declines of as much as 28.5 feet in the upper Alcovy River watershed, and 49.1 feet in the Redland–Pew Creek watershed.

Synoptic base-flow measurements were used to locate and quantify gains or losses to streamflow resulting from groundwater interaction (groundwater seepage). In September 2006, seepage gains were measured at five of nine reaches evaluated in the upper Alcovy River watershed, with losses in the other four. The four losing reaches were near the confluence of the Alcovy River and Cedar Creek where the stream gradient is low and bedrock is at or near the land surface. In the Redland–Pew Creek watershed, groundwater seepage gains were observed at each of the 10 reaches measured during September 2008.

Continuous specific conductance, temperature, and turbidity data were collected at gage sites located on Pew and Shoal Creeks, which drain about 32 percent of the city area, and at a background site on the Apalachee River located outside the city boundary. Continuous surface-water monitoring data indicate that reduced precipitation during 2006–2008 resulted in lower turbidity and higher stream temperature and specific conductance than in 2003–2005. In comparison to the other two stream sites, water at the Apalachee River site had the lowest mean and median values for specific conductance, and the greatest mean and median values for turbidity during October 2005–December 2008.

In addition to continuous water-quality monitoring, samples were collected periodically to determine fecal-coliform bacteria concentrations. None of the individual samples at the three sites exceeded the Georgia Environmental Protection Division (GaEPD) limit of 4,000 most probable number of colonies per 100 milliliters (MPN col/100 mL) for November through April. In the Redland–Pew Creek and Shoal Creek watersheds, the GaEPD 30-day geometric mean standard of 200 MPN col/100 mL for May–October was exceeded twice during two sampling periods in May–October 2007 and twice during two sampling periods in May–October 2008.

Groundwater studies conducted during 2003–2007 include the collection of borehole geophysical logs from four test wells drilled in the upper Alcovy River watershed to provide insight into subsurface geologic characteristics. A flowmeter survey was conducted in a well south of Rhodes Jordan Park to help assess the interconnection of the well with surface water and the effectiveness of a liner-packer assembly installed to eliminate that interconnection. At that same well, hydraulic packer tests were conducted in the open-hole section of the well, and water samples were collected to assess the depth and concentration of gross-alpha radiation detected in the well before and after well modification.

Revised and reposted April 26, 2010

First posted March 17, 2010