The authors introduce a method for evaluating natural reduction of microscopic particulates in aquifers that store groundwater under the direct influence of surface water. Many water utilities operate collection devices constructed in alluvial‐valley aquifers. Pumping groundwater from these systems may induce infiltration of surface water containing pathogenic protozoa. However, the porous sand and gravel of this kind of aquifer can significantly reduce the number of microscopic particulates that pass through the aquifer media. A method is proposed for evaluating the natural reduction efficiency of porous‐media aquifers, taking into consideration the transport of particulates through the aquifer during a period of maximum infiltration. The method allows regulators to estimate the risk of pathogenic protozoa, determine the log‐reduction credit warranted by the water's transport through the aquifer, and determine the type of treatment needed for the source.
The Central Wyoming Regional Water System operates 29 groundwater collection devices in an alluvial aquifer adjacent to the North Platte River. These collection devices were previously designated "groundwater under the direct influence of surface water." In lieu of constructing a conventional surface water treatment plant, the utility opted to perform a two-year study to demonstrate that riverbank filtration (i.e., natural filtration) provided the 2.0-log reduction of Cryptosporidium required under the Interim Enhanced Surface Water Treatment Rule (e.g., riverbank filtration is the primary filtration process). Using multiple surrogates of various sizes encompassing those of Giardia and Cryptosporidium, the utility demonstrated 2.0-log reductions in more than 90% of matched data pairs. No Giardia cysts or Cryptosporidium oocysts were found in any of the 170 groundwater samples. This level of removal was achieved during the summer months when infiltration rates are high because of increased river stage and well-field production.he Central Wyoming Regional Water System (CWRWS) uses the North Platte River alluvial aquifer as its primary source of drinking water. The utility operates 29 collection devices that provide 85% of the public water to the city of Casper and its surrounding communities. Specifically, the utility operates 25 vertical wells, with individual well capacity ranging from 0.5 to 1.0 mgd (1.9 to 3.8 ML/d); three horizontal (caisson) collector wells, each producing ~2.0 mgd (~7.6 ML/d); and one infiltration gallery, producing ~4.0 mgd (~15.1 ML/d). Maximum production from the well field is estimated to be 25 mgd (95 ML/d). The collection devices are scattered over 5 sq mi (13 km 2 ) in the Caspar and Morad well fields located on both sides of an oxbow adjacent to the North Platte River (Figure 1). Recharge to the aquifer is provided by the river and is supplemented to many of the collection devices T filtration 2005
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