Environ. Sci. Techno/. 1995, 29, 161-170
Impact of River Transport Characteristics on Contaminant Sampling Error and Design I A N G . D R O P P O * AND
C H R I S T I N A JASKOT National Water Research Institute, 867 Lakeshore Road, P. 0. Box 5050, Burlington, Ontario, Canada L7R 4A6
Within monitoring programs, loading errors are generally associated with the inadequate delineation of the temporal variance of discharge and of the parameter(s) of interest. Often little consideration is given to the impact of additional transport characteristics on contaminant sampling error and design. Detailed examination of five transport characteristics at a single river cross-section emphasizes the importance of understanding the complete transport/loading regime of a sampling station, defining the required end products of the monitoring program, and defining the accuracy required to meet specific program needs before implementing or evaluating a monitoring program. River transport characteristics examined are (a) contaminant transport modes, (b) short-term temporal and seasonal variability, (c)the relationship between dissolved and particulate contaminant concentrations and discharge, (d) load distribution with sediment particle size, and (e) spatial variability in a cross-section.
Introduction Understanding the qualitative state of a river basin’s water resources is a prerequisite for its effective management. Historically,monitoring programs have relied on infrequent chemical sampling protocols in conjunction with continuous water quantity monitoring to estimate river contaminant loads, which are as accurate as possible for a given sampling effort and cost. While loading errors generally are associated with the temporal variance of targeted contaminants and the relationships between discharge and concentration, little consideration is given to the impact of additional transport characteristics on contaminant loading estimation. The primary objective of this paper is to evaluate the impact of a river’s inherently variable transport characteristics on the sampling design of a monitoring program and on the accuracy of “representative” samples for tributary load estimation. This objective requires a knowledge of existing monitoring procedures and a knowledge of the transport characteristics which influence the loading regime of ariver. The latter requirement is, however,poorly understood. Therefore, the contaminant loading regime of a southern Ontario river was divided into five transport characteristics in order to address the many methodological and scientific issues which currently contribute to the uncertainty (in terms of accuracy) of contaminant loading estimations. The transport characteristics studied are (a) contaminant transport modes, (b) short-term temporal and seasonal variability at a site, (c) relationship between dissolved and particulate contaminant concentrations and discharge, (d)load distribution with sediment particle size, and (e)spatialvariability in a cross-section. Each river and sampling site is unique and requires individualized attention for the development of a monitoring program. The sitespecific transport characteristics investigated here should not be viewed as the norm and are used only as a demonstration of the requisite requirements for the effective development of flexible sampling strategies for the implementation or evaluation of monitoring programs.
Materials and Methods Samples were collected from a bridge 2 km downstream from the town ofAyr on the Nith River of southern Ontario. The sampling site receives only agricultural runoff and has no significant industrial or municipal point source discharges upstream of the site. The river transports primarily fine-grained sediment ( 63
42-63 31-42 19-31 12-19 8-12 '8
> 63
42-63 31-42 19-31 12-19 8-12 '8
bale) 37340 31345 21180 26903 30943 3 1020 48738 4057 1* 592 525 485 546 685 754 887 778" 50.3 322 77 107 103 118 52 81.2" 93.2 424 459 662 1160 1595 499 694*
Yo of
contribution to total S S b (@)
loadC (kdday)
Iron 0.0019 0.0022 0.0021 0.0021 0.0076 0.0062 0.0337 0.0592
145.27 141.21 91.08 297.48 481.55 393.82 3363.25 4918.12
2.95 2.87 1.85 6.05 9.79 8.01 68.38
Manganese 0.0019 0.0022 0.0021 0.0054 0.0076 0.0062 0.0337 0.0592
2.30 2.37 2.09 6.04 10.66 9.57 61.21 94.31
2.44 2.51 2.21 6.40 11.30 10.15 64.90
Lead 0.0019 0.0022 0.0021 0.0054 0.0076 0.0062 0.0337 0.0592
0.20 1.45 0.33 1.18 1.60 1.50 3.59 9.84
1.99 14.74 3.36 12.02 16.28 15.22 36.45
Copper 0.0019 0.0022 0.0021 0.0054 0.0076 0.0062 0.0337 0.0592
0.36 1.91 1.97 7.32 18.05 20.25 34.43 84.13
0.43 2.27 2.35 8.70 21.46 24.07 40.93
total loadd
a Individual fraction metal concentration determined by atomic adsorption (* = calculated bulk concentration). Individual fraction's contribution to total SS =total SS concentration x %fraction of total SS concentration. Fractional daily load (kglday) = ( a x b x discharge (23700 Us)x scale correction (8.64 x % oftotal load =fractional daily load/total daily load.
supply-dependent phenomenon (14, 24) (i.e., dependent on the grain size of material available for erosion and transport). For rivers with a transportable source of >63 pm material, a negative correlation between discharge and contaminant concentration generally results because the sample's chemical concentration will be diluted by an increase in larger size particles which possess lower chemical concentrations (8). The contaminant load generally however will increase due to the higher concentrations of SS transported. For rivers, such as the Nith River, which transport primarily
