Contaminants
Contaminants may be present in groundwater or surface water as a result of natural processes or through mechanisms of displacement and dispersal related to human activities. There are two types of contaminants, that of point and non-point sources. Contaminants from point sources discharge either into groundwater or surface water through an area that is small relative to the area or volume of the receiving water body. Examples of point sources include discharge from sewage treatment plants, leakage from fuel storage tanks, and seepage from landfills or septic tanks (Figure 1).
Figure 1. Transport of contaminants from a point source by groundwater can cause contamination of both surface water and groundwater resources (adopted from Winter et al. 1998)
Non-point sources introduce contaminants to the environment across areas that are large compared to point sources. Agricultural fields, in aggregate, represent large areas through which fertilisers and pesticides can be mobilised. Some pesticides are only slightly soluble in water and may attach to soil particles and are therefore less likely to cause contamination of water resources. Other pesticides, however, are detected in low, but significant, concentrations in both groundwater and surface water, such as atrazine (Squillace et al, 1993). Whether the initial contamination is present in groundwater or surface water is somewhat immaterial because the close interaction of the two can result in both being contaminated. Contaminants can migrate from industrial sites, land fills or septic tanks to reach waterways via the groundwater system. Likewise, leakage from polluted waterways can be a mechanism for contaminating valuable groundwater resources.
Recent studies indicate that fertiliser and pesticides applied to cropland can contaminate the underlying groundwater and then move along groundwater flow paths to surface water (Bolke and Denver, 1995). In addition, movement of agricultural chemicals between surface water and groundwater can be dynamic in response to factors such as bank storage during periods of high runoff and groundwater withdrawals.
Contaminants impacts on surface water and groundwater quality
The tendency for contaminants to move between groundwater and surface water is a key consideration in managing water resources. With an increasing emphasis on protecting water quality in catchments, coordination between river management and groundwater protection programs is essential to protect the quality of drinking water. Furthermore, groundwater and surface water interactions have a major role in affecting chemical and biological processes in lakes, streams and wetlands, which in turn affect water quality throughout the hydrologic cycle. Improved scientific understanding of the interconnections between hydrological and biogeochemical processes will be needed to remediate contaminated sites in both urban and rural environments.
Aquifers and surface water bodies need to be managed jointly to protect the quality of water resources. Understanding of the underlying processes of connectivity is needed when monitoring and predicting contamination plumes, establishing buffer zones, defining threshold loads, land-use planning and assessing development applications.
Case Studies
Relevant Links
CRC Water Quality and Treatment Consumer's Guide to Drinking Water
References
Squillace PJ, Thurman EM, Furlong ET, 1993. Groundwater as a nonpoint source of atrazine and deethylatrazine in a river during base flow conditions. Water Resources Research 29: 1719-1729.
Bolke JK Denver JM, 1995. Combined use of groundwater dating, chemical and isotopic analyses to resolve the history and fate of nitrate contamination in two agricultural watersheds. Water Resources Research 31: 2319-2337.
Winter TC, Harvey JW, Franke OL, Alley WM, 1998. Ground water and surface water - a single resource, U.S. Geological Survey Circular 1139, Denver, Colorado.