PRELIMINARY COMPARISON OF SAMPLING DENSITY AND FREQUENCY FOR SURVEYS OF SURFACE WATER QUALITY STUDIES: POTENTIAL DIFFERENCES IN RESULTS WITH WEEKLY, MONTHLY, AND 4WET/ 4DRY SAMPLING SCHEDULES

Sampling schedules for many watershed assessments of stream chemistry vary. Examples of USGS National NAWQA Program Circulars show sampling frequencies such as “once in 1996, monthly, monthly and more frequently, same as above plus weekly biweekly for 13 months”. Within some states sampling frequency for watershed assessment is 4 wet and 4 dry samples (a wet sample event is rainfall equal to or more than 0.1 inch in the 72 hours before sampling). An intense yearlong (January – December 2001) water quality study of 38 streams in Carroll and Heard Counties, Georgia coupled with our other studies (including weekly monitoring of DO, nitrate, and pH for up to 2.5 years on several streams) enable a comparison between different sampling frequencies. Land usage in the Piedmont of west Georgia is forested (silvicultural), agricultural (livestock grazing and poultry), suburban, urban, and forested wetland. The recent study in west Georgia (West Georgia Watershed Assessment) produced over 40,000 measurements of 21 physical, chemical, and microbiological water quality parameters at 70 sample stations. Twenty-eight stations were sampled weekly; the others were sampled as 6 wet and 6 dry.

Results indicate that streams in watersheds of second-growth forest that are closest to background water quality conditions can be monitored with minimal sample density and frequency. However, after timber harvest by clear cutting the monitoring plan needs to be modified to greater sample density and frequency. Watersheds in transition, such as agricultural and forestlands that are being transformed into urban and suburban sprawl require intense sample density and frequency. Our research on the Chattahoochee River in Carroll County (downstream of Greater Atlanta) indicates that monthly sampling provides good data for long-term water quality. However, weekly sampling of this impacted river is necessary to capture pollution accidents from Greater Atlanta. Sampling on a wet/dry basis can work well on watersheds that have potential non-point sources of pollution. Examples of such watersheds are urban, livestock grazing, and poultry with spreading of chicken litter. The wet sampling events must be aggressively pursued with sampling within 24 hours of significant rainfall.