Georgia Water Science Center

USGS Scientific Investigations Report 20085220

Methods to Evaluate Influence of Onsite Septic Wastewater-Treatment Systems on Base Flow in Selected Watersheds in Gwinnett County, Georgia, October 2007

This Web-only publication is available in pdf format (4 Mb): USGS SIR 20085220

Mark N. Landers and Paul D. Ankcorn

U.S. Geological Survey Scientific Investigations Report 20085220, 12 pages (Published December 2008)
Prepared in cooperation with the Georgia Environmental Protection Division

ABSTRACT

The influence of onsite septic wastewater-treatment systems (OWTS) on base-flow quantity needs to be understood to evaluate consumptive use of surface-water resources by OWTS. If the influence of OWTS on stream base flow can be measured and if the inflow to OWTS is known from water-use data, then water-budget approaches can be used to evaluate consumptive use. This report presents a method to evaluate the influence of OWTS on ground-water recharge and base-flow quantity. Base flow was measured in Gwinnett County, Georgia, during an extreme drought in October 2007 in 12 watersheds that have low densities of OWTS (22 to 96 per square mile) and 12 watersheds that have high densities (229 to 965 per square mile) of OWTS. Mean base-flow yield in the high-density OWTS watersheds is 90 percent greater than in the low-density OWTS watersheds. The density of OWTS is statistically significant (p-value less than 0.01) in relation to base-flow yield as well as specific conductance. Specific conductance of base flow increases with OWTS density, which may indicate influence from treated wastewater. The study results indicate considerable unexplained variation in measured base-flow yield for reasons that may include: unmeasured processes, a limited dataset, and measurement errors. Ground-water recharge from a high density of OWTS is assumed to be steady state from year to year so that the annual amount of increase in base flow from OWTS is expected to be constant. In dry years, however, OWTS contributions represent a larger percentage of natural base flow than in wet years. The approach of this study could be combined with water-use data and analyses to estimate consumptive use of OWTS.


CONTENTS

Abstract

Introduction

Purpose and Scope

Previous Studies

Study Areas

Hydrology of Onsite Septic Wastewater-Treatment Systems

Methods

Field-Measurement Methods

Influence of Onsite Septic Wastewater-Treatment Systems on Base Flow

Changes in Base-Flow Quantity and Quality

Relation of Onsite Septic Wastewater-Treatment Systems to Changes in Base Flow

Summary

Acknowledgments

References

 


REPORT AVAILABILITY

This Web-only publication is available in pdf format (4 Mb): USGS SIR 20085220
To view the PDF document, you need the Adobe Acrobat® Reader installed on your computer. (A free copy of the Acrobat® Reader may be downloaded from Adobe Systems Incorporated.)

Suggested citation:

Landers, M.N., and Ankcorn, P.D., 2008, Methods to evaluate influence of onsite septic wastewater-treatment systems on base flow in selected watersheds in Gwinnett County, Georgia, October 2007: U.S. Geological Survey Scientific Investigations Report 20085220, 12 p., available online at http://pubs.usgs.gov/sir/2008/5220/


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