Gaut, S., 2005: Factors influencing microbiological quality of groundwater from potable water supply wells in Norwegian crystalline bedrock aquifers. Doktor Ingeniør thesis 2005:99. Department of Geology and Mineral Resources Engineering, NTNU, 153 pp and appendices.
Microbiological analyses from 195 Norwegian waterworks based on groundwater in bedrock have been examined to study the vulnerability of bedrock wells to microbiological contamination. Inspections have been carried out at 49 of the 195 waterworks to identify possible causes to the recorded microbiological contamination. It is found that groundwater derived from bedrock wells is susceptible to microbiological contamination and needs better protection. Seasonal variations in the water quality occur. Coliforms are mostly detected from June to September. Cryptosporidium, but not Giardia, is detected in the groundwater from three of twenty waterworks. The microbiological water quality is correlated to (i) wellhead completion (including the well casing), (ii) type and thickness of superficial deposits, (iii) land use and contamination sources and (iv) distance from wells to running water. Recommended wellhead completion includes a well-house and a casing of at least 5.5 m, rising 40-50 cm above ground. The gap between casing and bedrock should be sealed. Wells are least vulnerable to microbiological contamination when the superficial deposits are > 2.5 m thick and the wells are located > 100 m from farmland and not within 75-125 m of running water. Variations in parameters, such as colour, turbidity, and iron, and high levels of total organic carbon can indicate that the aquifer or the well is vulnerable to microbiological contamination. Vulnerability mapping combined with a hygienic evaluation of the well area and delineation of protection zones based on simple analytical methods is suggested as a method to protect Norwegian bedrock wells.