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Systematic Stream Sampling Without Storm Chasing In Karst Basins

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  Pp. 141-153 in Total Maximum Daily Load (TMDL) Environmental Regulations–II Proceedings of the 8-12 November 2003 Conference (Albuquerque, New Mexico USA), Publication Date 8 November 2003.  .(doi:10.13031/2013.15550)
Authors:   Alex W. Fogle and Joseph L. Taraba
Keywords:   Mass load, systematic sampling, storm chasing, karst

Developing an adequate sampling and an appropriate mass load estimation method 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 but 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.

The study watershed lies within a karst terrain with streams that are shallow, emanate from springs, and flow over limestone bedrock. The basin hydrologic response is characterized as flashy and average discharge is 0.2 m3/s.

True nitrate-N and total solute mass loads were determined from continuously collected data and compared with mass loads determined from two systematic sampling techniques. The Method A utilized weekly, biweekly, and monthly grab sampling with instantaneous flow measurements. The Method B consisted of weekly, biweekly, and monthly grab sampling with continuous flow measurement. Method B was generally superior to method A in estimating mass loads. Method B consistently yielded percent differences less than 10%.

The systematic sampling successfully sampled all flow regimes and a comparison of grab sample concentrations with composite storm concentrations revealed that composite storm concentrations were essentially the same as grab sample concentrations. This phenomenon may be a characteristic of this karst system, which exhibits high infiltration rates and may have extensive mixing of new and old waters before flow reaches the springs. In this and similar karst systems, storm chasing is unnecessary to adequately estimate mass loads.

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