Water quality indices

Given the present constraints on capital expenditure for water quality improvements, it is essential that best management practices be adopted whenever possible. This research provides an evaluation of existing practices in use within the water industry for surface water quality classification and assesses water quality indices as an alternative method for monitoring trends in water quality. To this end, a new family of indices have been developed and evaluated and the management flexibility provided by their application has been examined. It is shown that water-quality indices allow the reduction of vast amounts of data on a range of determinand concentrations, to a single number in an objective and reproducible manner. This provides an accurate assessment of surface water quality which will be beneficial to the operational management of surface water quality. Previously developed water quality indices and classifications are reviewed and evaluated. Two main types of index are identified: biotic indices and chemical indices. The former are based exclusively upon biological determinands/indicators and are used extensively within the United Kingdom in the monitoring of surface water quality. The latter includes a consideration of both physico-chemical and biological determinands, but with an emphasis on the former variables. Their use is still the subject of much controversy and discussion. Four main approaches to the development of chemical indices can be identified in accordance with the aims and objectives of their design. Those developed for general application are known as General Water Quality Indices (WQIs) or Indices of Pollution, with the latter based predominantly upon determinands associated with man-made pollution. Those which reflect water quality in terms of its suitability for a specific use are termed use-related; whilst planning indices are those which attempt to highlight areas of high priority for remedial action on the basis of more wide-ranging determinands. The derivation and structure of previously developed indices have been evaluated and the merits and strengths of each index assessed. In this way, nine essential index characteristics were identified, including the need to develop an index in relation to legal standards or guidelines. In addition it was recognised that one requirement of an index should be to reflect potential water use and toxic water quality in addition to general quality as reflected by routinely monitored determinands. The development of river quality classifications within the United Kingdom is reviewed and the additional management flexibility afforded by the use of an index evaluated by comparing the results produced by the SOD (1976) Index with those of the National Water Council (NWC, 1977) Classification. The latter classification is that presently used to monitor water quality in Britain. The SOD Index was found to be biased towards waters of high quality and provided no indication of potential water use or toxic water quality. Nevertheless, it displayed a number of advantages over the NWC Classification in terms of the operational management of surface water quality. It was therefore decided to develop a new family of water quality indices, each based upon legally established water quality standards and guidelines for both routinely monitored and toxic determinands and each relating water quality to a range of potential water uses, thereby indicating economic gains or losses resulting from changes in quality. Four stages in the development of a water quality index are discussed: determinand selection; the development of determinand transformations and weightings; and the selection of appropriate aggregation functions. Four separate indices have been developed as a result of this research. These may be used either independently or in combination with one another where a complete assessment of water quality is required. The first of these is a General Water Quality Index (WQI) which reflects water quality in terms of a range of potential water uses. This index is based upon nine physico-chemical and biological determinands which are routinely monitored by the water authorities and river purification boards of England, Wales and Scotland. The second, the Potable Water Supply Index (PWSI) is based upon thirteen routinely monitored determinands, but reflects water quality exclusively in terms of its suitability for use in potable water supply (PWS). The two remaining indices, the Aquatic Toxicity (ATI) and Potable Sapidity (PSI) Indices are based upon toxic determinands such as heavy metals, pesticides and hydrocarbons which are potentially harmful to both human and aquatic life. Both indices are use-related, the former reflecting the suitability of water for the protection of fish and wildlife populations; the latter, the suitability of water for use in PWS. Each index is based upon nine and twelve toxic determinands respectively. These indices were developed in as objective and rigorous a manner as possible, utilising an intensive interview and questionnaire programme with members of both the water authorities and river purification boards. Rating curves were selected as the best way in which individual determinand concentrations could be transformed to the same scale. The scales selected for the WQI and PWSI are 10 - 100 and 0 - 100 respectively, whilst those of the ATI and PSI are 0 - 10. Each has been sub-divided in such a way as to indicate not only water quality, but also possible water use. Thus, the indices reflect both current and projected changes in the economic value of a water body which would occur as a result of the implementation of alternative management strategies. The curves were developed using published water quality standards and guidelines relating to specific water uses. Therefore, they contain information on standards which must be adhered to within the United Kingdom and this adds a further dimension to their management flexibility. Determinand weightings indicating the emphasis placed by water quality experts upon individual determinands were assigned to the determinands of the WQI and PWSI. However, weightings were omitted from the ATI and PSI due to the sporadic nature of pollution events associated with these determinands. These vary spatially and temporally, both in concentration and in terms of which determinand is found to be in violation of consent conditions. Therefore, on a national scale, no one determinand could be isolated as being more important than any other. Three aggregation formulae were evaluated for use within the developed indices: the weighted and unweighted versions of an arithmetic, modified arithmetic and multiplicative formulation. Each index was applied to data collected from a series of water quality monitoring bodies covering a range of water quality conditions. In each instance, the modified arithmetic formulation was found to produce index scores which agreed most closely with a predetermined standard, normally the classifications assigned using the NWC classification. In addition, this formulation produced scores which best covered the ascribed index range. However, the multiplicative unweighted formulation was retained for use within the ATI and PSI for the detection of zero index scores, i.e. when concentrations in excess of legal limits were recorded for these toxic determinands. The results from these studies validate the ability of each index to detect fluctuations in surface water quality. Therefore, the utility of the developed indices for the operational management of surface water quality was effectively demonstrated and the flexibility and advantages of an index approach in providing additional information upon which to base management decisions was highlighted. Amongst these advantages was the ability of an index to provide information upon which potential cost-benefit assessments could be made in relation to either spatial or temporal changes to surface water quality. Finally, the need for both general and use-related indices was investigated and found to be an advantage, although not strictly necessary, because the WQI efficiently recorded the range in quality conditions associated with the use of water in potable water supply.