The representativity index of a simple monitoring

Authors: Dominika Dąbrowska Rafał Kucharski Andrzej J Witkowski

Publish Date: 2016/04/25

Volume: 75, Issue: 9, Pages: 749-

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Abstract

The representativity index R u is a measure used in assessing the chemical status of groundwater based on monitoring studies This index is designed to describe the spatial homogeneity of the monitoring network The general formula for calculating the index R u includes the following parameters average distance to the nearest monitoring point number of monitoring points and size of the test area Calculations to determine the representativity index for four different shapes of the theoretical test study with the same area and the same number of monitoring points have been done These calculations suggest that the index value depends on the shape and the position of these points and it is less dependent on the size of the surface An assessment of the representativeness of the monitoring network for the different numbers and configurations of the piezometers around the TychyUrbanowice landfills based on the mentioned index has been done as well The best and the worst configurations of the monitoring network for these landfill sites in mathematical terms have been presented in this paper The results are surprising the highest index value is obtained with a single measurement point The calculations were performed with the area of landfill and the area limited by the range of piezometers as the exclusive test area To choose the optimal test area representativity indicator was calculated also for the monitoring network around waste landfills including the buffer network behind the piezometers The difference in the values of the representativity indicator for subsequent variants is astounding The representativity index for the same monitoring network is about 20  higher if we consider the test area limited by external piezometers and higher by another 20  taking into account the 95m buffer zone behind piezometers Due to increase of the representativity index value with a different width of buffer zone the mathematical calculations of the monitoring network’s representativeness should be supported by an analysis of the geological structure and hydrogeological conditions occurring in the analyzed areaThe monitoring and representativeness of a sampling of groundwater is a difficult and complex process resulting from the substantial spatial variability of the groundwater composition limited access to the aquifer and quite frequently a conceptually inadequate model of the monitoring system with poorly defined recharge and discharge zonesIt has to be mentioned that even in quite simple conditions with a limited spatial variability of groundwater composition the reliability of three downgradient piezometers can be not appropriate from the point of view of groundwater pollution risk assessment Witczak et al 2006The monitoring network design depends on the objectives of the monitoring The basic objective of groundwater quality monitoring is the detection and evaluation of changes in groundwater quality According to the Water Framework Directive WFD of the EU monitoring should control the input of pollutants into the groundwater locally point sources and control the status of all groundwater bodies In both cases proper monitoring network design should include the same factors eg geology hydrogeology recharge and discharge zones the migration time of contaminants land use and human impact regulatory requirements but could be different in some details eg the number and location of observation wells their construction the sampling frequency and the range of measurements taken according to environmental standards Witczak et al 2006 Different approaches have been proposed to establish a representative monitoring network for the abovementioned both cases but up to now there is no generally accepted methodology Nielsen 2006 Jousma and Roelofsen 2004 Grath et al 2001 Quevauviller et al 2009 For instance a minimum number of three sampling points in groundwater monitoring network per groundwater body or potential pollution source Grath et al 2001 Witkowski 2009 is requiredThe idea of this index is quite clear for homogenous network locations of monitoring points should be chosen in such a way that the distance from any point in the area to the nearest monitoring point is small relatively to the area of the test site Thus the higher the ratio the more homogenous the monitoring network is The role of the constant in the nominator is to normalize values of index to interval from 0 to 100  citing Grath et al 2001 “For a theoretical network with an optimal triangular pattern of sites the Representativity Index will be 100  For suboptimal less homogeneous networks the index will decrease” However for a theoretical example of circular area with single monitoring point in the center we obtain value of representativity index slightly exceeding 100  Dąbrowska et al 2013 Moreover it is recognized that the monitoring network is representative if R u ≥ 80  Rentier et al 2006

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