A GROUNDWATER VULNERABILITY ASSESSMENT OF THE GREATER PIETERMARITZBURG REGION USING DRASTIC IN A GIS ENVIRONMENT.

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1 A GROUNDWATER VULNERABILITY ASSESSMENT OF THE GREATER PIETERMARITZBURG REGION USING DRASTIC IN A GIS ENVIRONMENT. 1 Zwiers, A. S. and 1 Demlie, M. 1 University of KwaZulu Natal, School of Agricultural, Earth and Environmental Sciences, Department of Geological Sciences, Private Bag 54001, Durban, andyzwiers@gmail.com; demliem@ukzn.ac.za Abstract All groundwater is vulnerable to contamination, and natural inhomogeneity in the physical environment results in certain areas being more vulnerable to contamination than others. Inherent in the agricultural, domestic and industrial sectors of Pietermaritzburg, is the generation of contaminants which, upon reaching the aquifer, result in the deterioration of the quality of groundwater, thus resulting in the water no longer being fit for its intended use. The DRASTIC method is used to calculate the groundwater vulnerability of a 670 km 2 region including the city of Pietermaritzburg. The suggested ratings of each parameter are scrutinized and adapted, according to their relevance to the region and according to known geological occurrences. The use of this method enables the user to generate a regional scale vulnerability map of the ground-water in Pietermaritzburg. The vulnerability map generated has the ability to effectively highlight vulnerable areas to groundwater contamination, which is of critical importance in correct land-use planning, as well as in indicating areas of particular concern, where further detailed investigations are needed. The results of such an assessment are used as an input, together with a contamination inventory to assess the potential risk of groundwater pollution in a groundwater risk map. Furthermore, the result informs local decision makers and enables proactive prevention of groundwater pollution, in accordance with section 13 of the 1998 National Water Act. The intrinsic vulnerability of the Pietermaritburg region was found to range from low to very high. The area found to be highly vulnerable is the region northeast of Springbank which requires investigation at a local scale. Key words/phrases: Groundwater contamination, Intrinsic Groundwater Vulnerability Assesement, DRASTIC, Pietermaritzburg Region, GIS. 1. INTRODUCTION Due to the nature of modern human settlements and anthropogenic activities, there are various ways that water can become polluted. Contaminants originating from human activities have entered the subsurface environment through waste disposal practices, spills, industrial facilities and land application of chemicals (McCarthy and Zachara,1989). The most widely accepted definition of groundwater contamination is defined as 'the introduction into water of any substance in undesirable concentration not normally present in water, e.g. microorganisms, chemicals, waste or sewage, which renders the water unfit for its intended use (UNESCO, 1992). Pietermaritzburg is a large town with many well developed industrial and agrarian activities. Agricultural activities include the use of fertilizers which has great potential to cause nitrate pollution. This together with the wide use of pesticides to combat pests and diseases in areas where crops are being cultivated, results in agriculture being a very likely source of ground-water contamination. Cemeteries are problematic along with wastewater plants, tanneries, confined animal feeding operations. Transportation corridors should be included as well as the maintenance and fueling areas of the Oribi airport are regions of activity that contribute to groundwater pollution (U.S. EPA, 2003).

2 Furthermore, due to the concentration of large numbers of inhabitants in cities, the result will always be a potential for generation of contaminants that the environment cannot handle, for example at waste disposal sites. Of great concern in the informal settlements occurring in the area, are pit latrines, which can have detrimental effects that have been documented by Robins et al. (2007). Industrial solid and liquid waste can introduce a variety of metals, salts and organic contaminants into the ground. Leaking fuel tanks at petrol stations, and junkyards which contain materials such as grease, solvents and oil, pose a large contamination threat due to the nature of hydrocarbons (Zaporozec, 2002). Domestic sources can pose just as high a threat. Almost all sewerage pipes leak, often at a high rate, which results in nitrogen compounds and micro-organisms being emitted into the environment (Zaporozec, 2002). The occurrence of runoff in urban areas, that washes solid and liquid waste from roads, gardens and parks, down to depressions and water courses, is another threat to groundwater quality (Zaporozec, 2002). The groundwater pollution process occurs very rapidly in certain vulnerable areas; which are zones where the where there is potential for the rapid transfer of pollutants into the groundwater due to the soils, subsoils and bedrock not providing enough protection (Gogu and Dassargues, 2000). The concept of groundwaters vulnerability to contamination was first introduced by Albinet and Margat in Since then much progress has occurred to develop methods to highlight areas at high risk of being polluted and thus reveal areas that require groundwater protection (Gemitzi et al., 2005). This concept is based on the premise that the soil and rock above an aquifer is able to provide protection against contamination, to the groundwater below (Zaporozec, 2002). Variations such as shallow water tables, permeable soils and lack of overlying confinement that cause certain regions to be more susceptible than others to groundwater pollution, and are examples of vulnerable areas (Arthur et al. 2005). Although there are many geological conditions that either limit the movement of groundwater, or natural attenuation processes that reduce concentrations of pollutants, inhomogeneities in the earth result in some areas of groundwater being highly susceptible to contamination (Gogu and Dassargues, 2000). The aim of the DRASTIC methodology, is to highlight all the relevant parameters that affect groundwater pollution of an area and force careful consideration of these parameters. Instead of undertaking complex empirical methodologies at localised scales, the goal is to provide the preliminary guideline to respective parties, so that focused investigations can take place in a manner that optimizes the use of funds and resources as well as preventing groundwater quality problems. In terms of the scope of information generated by DRASTIC vulnerability map, there has been mixed opinion. It must be mentioned that groundwater vulnerability is a relative, non-measurable, dimensionless property (Vrba and Lipponen, 2007). However, by ranking vulnerability into categories, it can be used to produce a precautionary groundwater protection policy that is easily presented as a visual map thus facilitating communication (Vrba and Lipponen, 2007). Overlaying maps of these zones of vulnerability, with those showing or representing land uses that are potential sources of contamination will result in the formation of a risk map (Gogu and Dassargues., 2000). A ground-water vulnerability map is thus, a management and protection tool that will facilitate planning of human activities to help in reducing adverse impacts on the quality of groundwater, due to it being an essential economic resource. It is also a valuable tool, in assisting with the proper designation of protection zones (Robins et al. 2007). This is of great importance and complies with section 13 in the 4th chapter of the 1998 National Water act, which states that development should be prevented, where that development would pose an unacceptably high risk to the environment (DWAF, 2000). The DRASTIC model developed by the US EPA in 1989 has been selected as the most suitable model for the generation of such a tool. The guidelines stipulated by Aller et al. (1989) will be followed. Main research objectives

3 1. To create a GIS based output map, representing zones of susceptibility to groundwater pollution by contaminants, in the greater Pietermaritzburg area, using input layers corresponding to parameters influencing groundwater pollution. This map must have the ability to easily communicate information pertaining to groundwater vulnerability of the region, to the required end users. 2. To determine whether a regional groundwater vulnerability model is a suitable model to determine groundwater vulnerability, in the city of Pietermaritzburg and the surrounding region. 3. Make recommendations pertaining to the adaptions that can be made to the DRASTIC method, in terms of any factors that need to be included, and those that must be excluded to allow better accuracy. Location of the study area The study area encompasses the 2930CB topographic map sheet of areas locally surrounding the city of Pietermaritzburg, located 70 Km west of Durban in the KwaZulu-Natal province of South Africa. The greater Pietermaritzburg area, demarcating the study area for the groundwater vulnerability assessment covers an area of roughly 670 km2. The area lies within the latitudes co-ordinates 29 30'S and 29 45'S and longitudes 30 15'E and 30 30'E and encompasses the town of Pietermaritzburg in the central region as well as the Hilton, Wilgerfontien, Lynnefield Park and Springbank settlements comprising the major villages in the surrounding area which can be seen in figure 1 below. Figure 1. Location of study area together with the major rivers, roads, towns and topography of the greater Pietermaritzburg area modified from USGS, (DEM values in meters). 2. METHODOLOGY After reviewing the literature on groundwater vulnerability mapping, it was concluded that the most likely technique for use in the Pietermaritzburg and South African context, is the DRASTIC approach. DRASTIC is a weighting and rating index-type method that assesses vulnerability by means of seven parameters: depth of groundwater (D), net recharge (R), aquifer media (A), soil media (S), topography (T), impact of the vadose zone media (I) and the hydraulic conductivity of the aquifer (C) as seen in figure 2 below (Aller et al., 1987). These parameters are then assigned factor ratings by using a ten-grade relative scale describing the extent which they increase groundwater vulnerability. Thereafter, each of the parameters are multiplied by a qualitative-determined weighting coefficient, determined by the authors of the DRASTIC method (Aller et al., 1987). The intrinsic vulnerability is then calculated by the equation V(intrinsic)=D λd+r λr+a λa+s λs+t λt +I λi +C λc where λ is the weighting coefficient for each factor which will be described (Aller et al., 1987). This process is undertaken within the confines of a GIS environment using ArcGIS.

4 Figure 2. An overview of the DRASTIC methodology The depth to groundwater parameter is best sourced from the South African department of water affairs. The Groundwater Resources Information Project (GRIP) and National Groundwater Archive (NGA) databases have been used after sourcing them from personnel within the department. The spatially varying recharge values were used per quaternary catchments, sourced from the department of water affairs. The data comes from the recharge dataset of the Groundwater Resource Assessment (GRAII) program completed in June This data describes the amount of net recharge in millimeters per annum. This data is then prescribed a rating according to the table below. The ratings for the aquifer media are assigned according to Lynch et al. (1996) as they provide a relevant comparison of rocks in a South African context. The hydrogeological map of the study areas and their descriptions gave an overview of the aquifers present in the study area. This map was created by the department of water affairs in 1998 at a scale of 1: thus, the aquifers in the region are not accurately represented in terms of composition and spatial boundaries. It was initially assumed that the various aquifer classes were grouped together as single units, whereby in reality these units are actually separate entities. Utilizing a 1: geological map together with the information on how the mapping process was undertaken, given by DWAF (1995), these units were more precisely constrained. The parameters involved in the designation of ratings to soil is based on particle size and the soil depth. Two databases were used namely SOTERSAF 1: ( part of the global Soils and Terrain database (SOTER) 1: ) and AGIS (ARC-ISCW, 2003) have uniquely described spatial data. The former describes soil in terms of size classification while the latter describing soil in terms of pedological development. By combining the two databases, a picture of the soil could be generated in terms of the major soil types as well as the areas of limited soil formation. The topography data is sourced from the Shuttle Radar Topography Mission (SRTM) 90m dataset from the United States Geological Survey. After calculating and mapping the percent slope values, they were assigned ratings based on the corresponding ranges defined by DRASTIC as seen in table 4.6 (Aller et al., 1987). The vadose zones for each hydrogeologic unit are prescribed according to the geology modified from the 1: series geological map, sourced from the Council for Geosciences (2002). Hydraulic conductivity of the aquifer is the most difficult property to determine and is the parameter that is most prone to error (Stigter et al., 2006). This is particularly true in South African hydrogeologic literature, where borehole yields are widely reported instead of hydraulic conductivity. Hydraulic conductivities can be located for Karroo aquifers in other parts of the country however Lynch et al. (1994) has reported that there is a high amount of variability of hydraulic conductivities that occur in South Africa. It was due to this fact, and in addition to the limited information available, that this parameter was left out of the country-wide drastic equation until further information becomes available (Lynch et al., 1994).

5 Conversion of all the input parameters into 25m GRID rasters can be undertaken followed by a weighted sum overlay operation. However, a more accurate way of undertaking the overlay is within the confines of an edit session, and utilizing zonal statistics in highly heterogeneous zones, ratings are assigned to every hydrogeologic unit for each parameter. The final DRASTIC output map is a created field that is calculated using the field calculator. 3. RESULTS AND DISCUSSION The final DRASTIC vulnerability index is classified into 5 equal intervals which range from extremely low to extremely high, in accordance with authors such as Vias et al. (2005). The input parameters, in the form of GIS layers, are included in the appendix. Most of the region is represented by a low value of vulnerability, most importantly the area immediately surrounding the city of Pietermaritzburg. Slightly elevated values are calculated for the areas enveloping the Holocene deposits and Volkrust formations. The dolerite intrusions although variable, coincide with elevated values of vulnerability. The most vulnerable area is the north west region above Springbank which is on average, a whole class higher than the other regions in figure 3. Figure 3. Vulnerability index values calculated for the various hydrogeological units found in the greater Pietermaritzburg area utilizing equal interval, 5 class classification The results show regions of vulnerability of the various hydrogeologic units, to pollution, which can be used for the generation of precautionary ground-water policies. Each method of vulnerability assessment results in distinct output maps, although in certain areas they show similar distributions of vulnerability. The DRASTIC method undertakes the intrinsic prediction independent of the type, and way in which the contaminant is loaded. These properties, describing aspects of contamination, are however, critical when applying the groundwater vulnerability map to analyze the potential risk to groundwater pollution of a region. Areas of low vulnerability are only vulnerable to the most persistent pollutants and over a long term while the moderate classes are vulnerable to many types of pollutants, except those that are easily transformed and/or greatly absorbed (Morris et al., 2003). The highly vulnerable regions are vulnerable to most water borne pollutants that have a rapid impact in the short term (Morris et al., 2003). Thus, an understanding both of the contaminants present, and the subterranean processes affecting these contaminants, must be known. An example where information is critical is in biological contaminants. The

6 deep weathering and high porosity of volcanic igneous rocks has rightly been prescribed a high rating for that parameter. In terms of biological contaminants however, the thick soils formed by this process cause sufficient filtration of pathogens and thus are a factor reducing the potential for groundwater pollution of the area (Davis and DeWiest, 1966). Thus, for a sound investigation, a team of highly qualified professionals is required, and all possible facets should be explored, utilizing information from a contamination inventory. A great importance lies in the understanding of which contaminants are present in a region. This is far more complex than describing the various land uses found in an area as there are myriads of sources and causes of groundwater contamination, and are as diverse as human activities (Zaporozec and Miller, 2000). In order to evaluate the existing or potential impacts of human activities on groundwater, it is necessary to document, all existing and potential sources of contamination in terms of their location, type, characteristics, and estimated magnitude of impact on groundwater as well as the contamination already caused by the existing sources. The process whereby this is achieved, is known as a contamination source inventory and is well described by Zaporozec (2002). 4. CONCLUSION In conclusion, the DRASTIC method is a highly effective groundwater vulnerability assessment technique that can be proficiently used to determine the intrinsic vulnerability of areas at a regional scale. The parameters rating and weights have been well designed and are applicable to many situations however, knowledge of the processes occurring in the subsurface is critical for the correct assignment of the ratings. The hydraulic conductivity parameter should be excluded on a regional scale due to limitations in the available data as well as the high variability that occurs even within a single lithological unit. The GIS environment in ArcMAP is highly effective for the analysis involved in both the DRASTIC and Susceptibility Index methods used as well as efficiently handling large amounts of spatial data. Groundwater vulnerability maps are clearly useful in land-use planning investigations highlighting areas where contamination could potentially occur. The methodology has enabled a greater understanding of the vulnerability to pollution of the groundwater regime in the greater Pietermaritzburg region. Furthermore, the output DRASTIC map has the ability to easily communicate information pertaining to groundwater vulnerability of the region, to the required end users. This is of critical importance if chapters 3 and 4 of the National Water Act are to be fully realized. Recommendations The calibration of the DRASTIC model is a prominent occurrence in groundwater vulnerability literature. There have been many studies undertaken whereby correlation with measured pollutants and predicted vulnerability has been found to be a highly advantageous procedure (Rupert, 2001; Panagopoulos et al., 2006). The data available from the department of water affairs and forestry is very poor in terms of water chemistry. Thus, highly accurate sampling of the region and assessment of the correlation of each parameter to nitrate concentration is required if the next stage of the vulnerability assessment is to be fully realized. A detailed investigation involving the complete DRASTIC method should be undertaken in the regions of highest categories in the vulnerability and susceptibility maps, utilizing pits, core logs and boreholes in which detailed tests such as permeability and recharge can be more precisely calculated. A groundwater inventory should also be compiled in the greater Pietermaritzburg region according to the guidelines set out by UNESCO (2002), starting with the regions of highest vulnerability in the case that an immediate threat needs to be ameliorated (Zaporozec, 2002). References Aller, L., Bennett, T., Lehr, J.H., Petty, R.J., Hackett, G. (1987) DRASTIC: a standardized system

7 for evaluating ground water pollution potential using hydrogeologic settings. EPA-600/ , EPA, Washington, DC. Davis, S. N. and DeWiest, R. J. M. (1966) Ground Water In Igneous and Metamorphic Rocks. Hydrogeology. John Wiley and Sons, New York, 463. Lynch, S.D., A.G. Reynders, and R.R. Schulz. (1994). Preparing input data for a national-scale groundwater vulnerability map of Southern Africa. Water S.A, Pietermaritzburg, v. 20, McCarthy, J. F and Zachara, J. M. (1989) Subsurface transport of contaminants. Environment. Science and Technology, American Chemical Society, v. 23, n 5. Morris, B.L., Lawrence, A.R., Chilton, P.J., Adams, B., Calow, R.C., Klinck, B.A., (2003). Groundwater and its susceptibility to degradation, a global assessment of the problem and options for management. United Nations Environment Program, Nairobi. Panagopolous, G.P., Antonakos A. K., and Lambrakis N. J. (2006) Optimization of the DRASTIC method for groundwater vulnerability assessment via the use of simple statistical methods and GIS. Hydrogeology journal, v. 14, n. 6, Robins, N.S., Chilton, P.J., Cobbing, J. E. (2007) Adapting existing experience with aquifer vulnerability and groundwater protection for Africa. Journal of African Earth Sciences. Elsevier. v. 47, Rupert, M. (2001) Calibration of the DRASTIC Ground Water Vulnerability Mapping Method. Groundwater. USGS, Colorado. v. 39, n. 4, p Stigter, T. Y., Ribeiro, L., Carvalho-Dill, A. M. M (2006) Evaluation of an intrinsic and a specific vulnerability assessment method in comparison with groundwater salinisation and nitrate contamination levels in two agricultural regions in the south of Portugal. Hydrogeology. Springer- Verlag, Berlin. v. 14, U.S. Environmental Protection Agency (2003). Potential sources of drinking water contamination index, documentation from webpage: Vias, J.M., Andreo, B., Perles, M. J., Carrasco, F. (2005) A comparative study of four schemes for groundwater vulnerability mapping in a diffuse flow carbonate aquifer under Mediterranean climatic conditions. Environmental Geology. Springer-Verlag. n. 47, Vrba, J and Lipponen, A. (2007) Groundwater Resources Sustainability. IHP-VI, Series on Groundwater. UNESCO, Paris. n. 14. Zaporozec, A. (2002) Groundwater contamination inventory. IHP-VI, Series on Groundwater. International Hydrological Program, UNESCO, Paris. n. 2. Zaporozec, A. and Miller, J. C. (2000). Ground-Water Pollution. UNESCO, Paris APPENDICES

8 Appendix 1a) Results of the IDW geostatistics analysis of depth to groundwater and 1b) Ratings assigned to the classes Appendix 2a. Showing the amount of recharge occurring in each quaternary catchment and 2b. Showing the recharge ratings assigned to the recharge regions. Appendix 3a. Showing the occurrence of aquifer types and 3b. Showing the ratings assigned to them in the study area.

9 Appendix 4a. Soil types found in the region and 4b. The ratings assigned to those soils. Appendix 5a. Results of the slope analysis and 5b. The rating given to the corresponding regions. Appendix 6a. Vadose zone material found in the 2930CB area and 6b. The ratings pertaining to the impact of the vadose zone of that area.