In 1989 the EPA promulgated the Surface Water Treatment Rule (SWTR) in response to the 1986 Amendments to the Safe Drinking Water Act (SDWA). The overall goal of the SWTR is to protect consumers from pathogens. The SWTR recommends a multiple-barrier approach including source water protection, filtration and disinfection when surface water is used as source water for drinking.
The requirements of the SWTR also apply to ground water under the direct influence (GWUDf) of surface water which may include vertical and horizontal wells, infiltration galleries, springs, etc. The SWTR required the evaluation of all community ground water sources for GWUDI by June 29, 1994 and all non-community ground water sources by June 29,1999.
Typically, GWUDI status is determined by state authorities using a combination of hydro-geologic criteria, sanitary surveys, well construction logs and analytical testing. Testing may attempt to prove or disprove direct hydraulic connection between surface and ground waters by correlating shifts in conductivity, temperature. pH, etc., and may include Microscopic Particulate Analysis (MPA). Water supplies identified as being GWUDI must comply with requirements of the SWTR and treat their source(s) accordingly.
EPA Consensus Method
The EPA Consensus Method for MP A is based on identifying surface water 'bioindicators' such as plant debris, algae, diatoms, insects, rotifers. Giardia, and coccidia which are characteristic of surface waters. In the Consensus Method, risk scores are assigned to each category of bioindicators. After tabulation of the number and type of bioindicators observed in a particular sample, an overall risk rating score is calculated. This score indicates the risk of surface water contamination.
The MPA method involves filtering a minimum of 500 gallons of ground water through a 1 micron (jim) nominal-pore-size fiber wound filter over a maximum 24-hour period. The filter is then processed in the laboratory by eluting the particles from the fibers, concentrating the eluant, and microscopically examining slides for bioindicators. The bioindicators are quantified and used to calculate a relative surface water risk factor as described above.