WATER QUALITY SAMPLING SCHEMES FOR VARIABLE FLOW CANALS AT REMOTE SITES¹

Abstract

Growing interest in water quality has resulted in the development of monitoring networks and intensive sampling for various constituents. Common purposes are regulatory, source and sink understanding, and trend observations. Water quality monitoring involves monitoring system design; sampling site instrumentation; and sampling, analysis, quality control, and assurance. Sampling is a process to gather information with the least cost and least error. Various water quality sampling schemes have been applied for different sampling objectives and time frames. In this study, a flow proportional composite sampling scheme is applied to variable flow remote canals where the flow rate is not known a priori. In this scheme, historical weekly flow data are analyzed to develop high flow and low flow sampling trigger volumes for auto‐samplers. The median flow is used to estimate low flow sampling trigger volume and the five percent exceedence probability flow is used for high flow sampling trigger volume. A computer simulation of high resolution sampling is used to demonstrate the comparative bias in load estimation and operational cost among four sampling schemes. Weekly flow proportional composite auto‐sampling resulted in the least bias in load estimation with competitive operational cost compared to daily grab, weekly grab sampling and time proportional auto‐sampling.

Comparison of sampling methodologies for nutrient monitoring in streams: uncertainties, costs and implications for mitigation

“…However, in dynamic systems such as streams, flow-proportional composite sampling is conceptually a better approach to estimate nutrient fluxes (Abtew and Powell, 2004;Ort et al, 2010). The advantages of flow-proportional composite sampling versus time-proportional composite sampling were compared in a study conducted in three small-sized streams in Norway (Haraldsen and Stålnacke, 2006).…”

Comparison of sampling methodologies for nutrient monitoring in streams: uncertainties, costs and implications for mitigation

“…The monitoring station at Odderbaek was placed near the mouth of the stream before it flows into Lake Kulsø (latitude 55.932 • N, longitude 9.310 • E). Upstream of the station, a 1 km stretch was restored in December 2010 by raising the stream bed, creating meanders and disconnecting tile drains (Audet et al, 2013). The Gelbaek station was positioned at Lyngby Bridge (lat.…”

“…In the present study, time-proportional composite sampling was used as the best estimate of the true load. However, in dynamic systems such as streams, flow-proportional composite sampling is conceptually a better approach to estimate nutrient fluxes (Abtew and Powell, 2004;Ort et al, 2010). The advantages of flow-proportional composite sampling versus time-proportional composite sampling were compared in a study conducted in three small-sized streams in Norway (Haraldsen and Stålnacke, 2006).…”

Development and Demonstration of a Free Surface Flow‐Dependent Sampling Technology

“…To determine nutrient loads and verify compliance with state water quality standards or permit criteria, SFWMD collects water samples at many water control structures. Abtew and Powell (2003) discussed the various sampling schemes used throughout the Central and South Florida Flood Control District geographic regions and worldwide. Water at most structures is sampled by a ''single surface water grab method'' collected at 0.5 m depth at a predetermined frequency.…”