Natural protection of groundwater against bacteria of fecal origin

A survey of rural groundwater quality in Canada showed that some drinking water wells were consistently contaminated with bacteria, whereas others were consistently free of such contamination. The purpose of this study was to identify profile characteristics that make wells susceptible to, or provide natural protection against microbial contamination. Information from surveys was supplemented with information from regional maps of soil, physiography, quaternary and bedrock geology, together with descriptions of stratigraphy from the well driller's records. The prediction of the most relevant factors affecting well water quality in Ontario was evaluated for the different soil and geological profiles in the Goromonzi district of Zimbabwe. High risk wells in Ontario were located most often at sites with older limestone or dolostone bedrock, and in clay or clay loam soil. The presence of a sandy soil may offer some protection to groundwater resources in very vulnerable dug or bored wells. The presence of shale and hardpan layers may offer protection due to the impermeable nature of these deposits. Well construction was a dominant factor affecting water quality. Shallow dug or bored wells, located in sites where there is a thin soil profile or shallow water table, can be very vulnerable to contamination. Dug or bond wells were most affected by the soil and geological setting of the well location. Manure spreading, particularly daily manure application, increased the risk of well water contamination, whereas other farming practices such as tillage were less predictive of well vulnerability. Knowledge of the history of contamination for a well was useful in predicting well susceptibility, A brief assessment of topography and local surface water provided sufficient information to indicate the direction of groundwater now, the most important feature that can be used to predict the susceptibility of a well in a given location. (C) 2000 Elsevier Science B.V. All rights reserved.