SYSTEMATIC SAMPLING AND STORM CHASING IN KARST BASINS

A wide variety of approaches have been proposed and utilized to quantify the pollutant load flowing past a fixed point on a stream. Obtaining satisfactory results from mass load estimators require adequate sampling strategies. Developing an adequate sampling and mass load estimation method appropriate for a given stream and a given constituent has been stated to be difficult, due to inconsistencies arising from both hydrologic and constituent characteristics. Systematic sampling supplemented by storm chasing has been reported to be the most effective strategy for estimating annual loads, but the approach could still result in load overestimations of 25–50%. Systematic sampling with little or no storm chasing is often the strategy of choice due to budgetary and manpower constraints.The objective of this study was to determine if storm chasing is necessary in making mass load estimations in small karst basins. “True” nitrate-nitrogen and total solute mass loads were determined from continuously collected data and compared with mass loads determined from two systematic sampling techniques. The first method (method A) utilized weekly, biweekly, and monthly grab sampling in conjunction with instantaneous flow measurements. The second (method B) consisted of weekly, biweekly, and monthly grab sampling with continuous flow measurement (10-minute intervals). Continuous flow data was integrated to obtain the total flow volume represented by each grab sample.A comparison of “true” loads with loads produced by each of the techniques revealed that within this basin, storm chasing was not necessary. All percent differences between load estimates and the true load were less than 20%. Method B was generally superior to method A in estimating mass loads. Method B consistently yielded percent differences less than 10% and often less than 5%. Monthly grab sampling with continuous flow measurement produced satisfactory mass load estimations without the expense and effort of storm chasing. Method A produced very satisfactory results with biweekly sampling and instantaneous flow measurement. Storm sampling is not required because it was found that the flowcomposited storm nitrate-N and total solute sample concentrations were comparable to the systematically collected grab sample concentrations both in value and trend.