Groundwater value rating

Groundwater is difficult to value; and as a result, economic valuation and future considerations have historically played almost no part in decision-making. Most groundwater studies to date have focussed only on the valuation of limited production-related services provided by ground-water, and not on a more comprehensive review of production and ecological services (NRC, 1997). A fundamental step in valuing groundwater resources is recognising and quantifying the resource’s total economic value. Knowing the resource’s total economic value is crucial for deter-mining the net benefits of policies and management actions. The total economic value comprises

‘extractive value’ and ‘in-situ value’. Extractive values are derived from municipal, industrial, commercial, and agricultural demands met by groundwater. In-situ values include, for example, the capacity of groundwater to 1) buffer against periodic shortages in surface water supplies;

2) prevent or minimise subsidence of the land surface from groundwater abstraction; 3) protect against seawater intrusion; 4) protect water quality by maintaining the capacity to dilute and assimilate groundwater contaminants; 5) facilitate habitat and ecological diversity; and 6) provide discharge to support recreational activities.

Groundwater contamination inventory

FIGURE6.4 The matrix system used for groundwater vulnerability classification on Map 2 - Matlock, England (Source:Palmer, 1988)

It is important to recognise the value even when one cannot develop specific quantitative separations of the various components. Even a partial or inexact measurement of economic value can greatly aid decision-making by providing insight into how economic value changes with a policy or management decision. The reader is referred to the work of the U.S. National Research Council (NRC, 1997) for more detail on valuing groundwater. Groundwater valuation is a complex process and a number of issues with high uncertainty have to be addressed. The groundwater value rating is presented in this guideline as an example of the evaluation of an important component of the groundwater contamination risk map, although we recognise that it includes an incomplete process. The approach is based on aquifer classification as a basis for groundwater value rating. Usually such a rating is developed using a matrix combining an aquifer classification system and a second, user-defined variable.

(i) Aquifer classification system

Parsons (1995) reviewed international aquifer classification systems and then developed a system for South Africa. The following discussion is adapted from his report. An aquifer classification system may be developed for a number of reasons. On a general and national scale these include:

prioritising aquifers, developing and implementing policies and regulations, allocating limited groundwater management resources, setting water quality standards, defining monitoring requirements, and educating the general public. More detailed applications at a larger scale could include: providing information for policy formulation and implementation, setting of controls regarding groundwater abstraction, regional physical and land use planning, urban zoning, and setting of permitting and siting requirements.

On evaluating aquifer classifications used elsewhere in the world, a number of common features were readily apparent. Most systems use between 3 and 5 classes; a wide range of criteria are used, many of which are related to aspects of aquifer usage considerations as opposed to the physical characteristics of the aquifer itself; the classifications tend to be quite general, with only total dissolved solids content being used quite specifically; and most systems appeared to be geared toward general decision-making (national scale) with more detail being required for more focussed judgments (local scale).

Three levels of aquifer systems emerged from the literature, namely: highly productive aquifers; aquifers of lower potential, but of local importance to their existing or potential user; and poor aquifers. This grouping conforms to that of the British Geological Survey (NRA, 1992).

Parsons (1995) noted that there were a number of positive attributes making this equally applicable to South Africa, especially that the grouping can accommodate both major and minor aquifersas well aspoor aquifers. In addition to these three, he included sole source aquifersand special aquifer systemsin his classification system. These five types of aquifers are shown schematically in Fig. 6.5.

The aquifer classification scheme described above has been developed for South Africa, which has mainly fractured rock aquifers and minor primary porosity aquifers. It focuses primaril on abstraction value. However, the classification could also include water use, water quality, or ecological value of groundwater, for example, spring or river baseflow contribution or support of ecosystems. In any case, a classification system should reflect the situation that exists at the study

FIGURE6.5 South African aquifer classification system (Source:Parsons, 1995)

Major Aquifer System

site. Such a system can be either the existing aquifer classification scheme for that country, an adaptation if the scheme is not quite suitable, or a new scheme.

There are a number of other groundwater resource or aquifer classification systems. For example, the state of Connecticut in USA adopted in 1980 ‘Water quality standards and criteria’, which included a groundwater classification system (NRC, 1986). Connecticut is generally underlain by a shallow bedrock and water wells are commonly drilled in the alluvial aquifers to depths less than 30 m. Development of the system was prompted by concerns about the effects of wastewater discharges on groundwater. The system is based on use standards rather than quality standards. All of the state’s groundwater was classified into one of the four classes established (Table 6.2).

The use standards are intended to restore or maintain the quality of groundwater to a quality consistent with its use for drinking without treatment. All groundwater shall be restored to the extent possible to a quality consistent with class GA, except where groundwater is designated as class GB or class GC. Other examples of aquifer/groundwater classification systems developed by the states in USA are summarised in Pye et al. (1983).

(ii) Groundwater value rating

Once an aquifer classification system has been defined (para. 6.2.3 (i)), then a rating of the ground-water value can be assigned. Table 6.3 shows two possible ratings for the South African aquifer classification system, which is described in the previous section. A sole source aquifer is given the highest rating because of the consequences to the dependant community should the aquifer be

‘lost’ for use. A special aquifer system can be any of the aquifer classes; it is merely an aquifer that Groundwater contamination inventory

TABLE6.2 Groundwater classification system of the state of Connecticut, USA (Source:NRC, 1986)

Class

May not be suitable for potable use unless treated.

More suitable for receiving permitted discharges then for development of public or private water supply.

Discharge allowed

Wastewater of human or animal origin and other minor cooling and clean water discharges.

Wastewater of predominantly human, animal, or natural origin that poses no threat to untreated drinking water supplies.

All the above plus certain treated industrial wastewater, which shall not cause degradation of groundwater that could preclude its future use for drinking without treatment.

All the above plus other industrial discharges that do not result in surface water quality degradation below established goals.

TABLE6.3 Example of two possible ratings applied to the South African aquifer classification system

Aquifer class

has a differing legal status because it has been designated as such by ministerial decree of the Minister of Water Affairs.