In general, commercial laboratories show a lack of proficiency in testing for Giardia and Cryptosporidium. The authors conducted a blind survey of 16 commercial laboratories in order to evaluate their ability to test for Giardia and Cyptosporidium using the American Society for Testing and Materials (ASTM) method. After filters were spiked with approximately 740 Giardia cysts and Cyptosporidium oocysts or 500 cells of the alga Oocystis, they were sent to the laboratories for analysis. For the laboratories that submitted reports, Giardia cyst recoveries ranged from 0.8 to 22.3 percent, averaging 9.1 percent; Cyptosporidium oocyst recoveries ranged from 1.3 to 5.5 percent, averaging 2.8 percent. Four of the laboratories falsely reported the Oocystis as positive, four laboratories failed to recover the Giardia cysts, and six laboratories failed to recover the Cryptosporidium oocysts. Not all laboratories strictly followed the ASTM method. The majority of laboratories need to make improvements in one or more of the following areas: response to clients, adequacy of sampling equipment and directions for use, analytical methods, and data accuracy.

Ten production wells drilled in a highly productive sand and gravel aquifer and recharged by an adjacent river were evaluated for potential Giardia and Cryptosporidium contamination. The goal of this study was to determine whether riverbank filtration could achieve significant reduction to a level at which no additional engineered filtration would be required for pathogenic protozoa. Pathogen monitoring was conducted sporadically over 10 years. Intensive monitoring was conducted for a 20‐month period at 10 “flowpath wells” and two production wells. Algae, diatoms, and other surface water indicators were found in 57% of 128 groundwater samples. Of 285 groundwater samples collected and analyzed for Giardiao or Cryptosporidium, no pathogens were detected. No correlation existed between Giardia, Cryptosporidium, and surface water indicators. All surrogates demonstrated a minimum 4‐log reduction. Even though there is hydrologic influence, riverbank filtration is highly effective in removing pathogenic protozoa.

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.

Riverbank filtration relies on the streambed and aquifer matrixes to reduce pathogens under varying conditions of induced infiltration resulting from changes in river stage and flow velocity. The Greater Cincinnati (Ohio) Water Works monitored hydrologic parameters and water quality at its Charles M. Bolton well field during a comprehensive flowpath study. The study determined the frequency of occurrence of high river-stage events from historical data and monitored hydrologic parameters to estimate the potential unit infiltration rate. Giardia, Cryptosporidium, algae, spores, particle counts, and turbidity were also monitored. The project investigated potential pathogen/surrogate breakthrough during several high river-stage and infiltration events. High-stage events occurred less than 4% of the time. Giardia and Cryptosporidium were not detected in any groundwater samples. Increases in surrogate concentrations were minimal and maintained >3.5-log reduction. The streambed and aquifer have the ability to buffer water quality effects from major increases in the infiltration rate.

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