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Groundwater Under the Influence

During the past 30 years giardiasis has been recognized as one of the most frequently occurring waterborne diseases in the United States.

The occurrence and detection of this parasite (Giardia) and drinking water source identification and protection has become a matter of urgent concern to those responsible for water utility operations in endemic areas. Because of these concerns the Surface Water Filtration Rule was established and protocols were developed for determining if a source was characterized as surface water, groundwater, or groundwater under the influence of surface water. These procedures are called the SWIP - Surface Water Investigation Protocol and the MET - Microscopic Evaluation Technique. During the MET, the number of a variety of surface water indicators (algae, diatoms, organic debris, plant material, crustacea, insects, larvae, etc) are identified and enumerated.

Mr. Brian Oram has been involved with conducting over 500 SWIP, MPA/MET, Wellhead Protection Studies (Zone I, II, and III), and Groundwater Under the Influence Investigations. If you would like more information, please Contact Mr. Brian Oram, Professional Geologist.‍

Evaluation - A Three-Step Process

Evaluation of groundwater sources (springs, wells, etc) includes the determination of the degree of influence from a surface water source. For the Groundwater Under the Influence evaluations, a three-step process is recommended. The steps include:

1 Initial evaluation of hydrogeological settling, water quality, and a MET Particulate Analysis. Review well construction logs, camera survey the well, and monitor static and dynamic water levels.  It may be advisable to modify the well to better isolate the primary water-bearing zone.

2 Long-Term daily monitoring including static and dynamic water levels and general water quality such as: pH, turbidity, temperature, ORP, standard plate count, T. Coliform, and E coli.
3 Follow-up water quality and particulate analysis during the conditions when the source is most vulnerable to surface water influence.

Step 1

The first step is an initial sampling of the source following a major recharge event, such as a heavy rainfall (> 2 in/wk) and one sample collected during the summer following an extended period of little or no rainfall. The data are evaluated using the MET Technique. The Microscopic Evaluation Technique (MET) involves the identification, sizing, and population estimates of microorganisms and organic or inorganic debris found in water. The results of the laboratory data are then used in a series of risk assessment tables to establish the potential risk of surface water influence. In addition to the MET, a preliminary hydrological assessment is conducted to determine the potential sources of contamination, possible routes of contamination, and overall security of the source.

Step 2

The second stage is daily monitoring of the source for temperature, pH, turbidity, conductivity, static water level, pumping rate, flow rate, or any other relevant parameters. In addition, routine bacterial analysis is performed and climatic data is recorded. This monitoring is conducted for a period of 6 months or more. Following the 6 month period, the data are evaluated.

Step 3

If a trend is observed, such as an increase in temperature or turbidity a few days following a recharge event, the source is then sampled using the MET procedure during the time when the source is most vulnerable to influence.

PA State Protocol (Link to "Summary of Key Requirements for SWIP.pdf")

Recommendations For Conducting a MPA / MET for a Well

Samples should be collected during the last 12-hour period of a typical 72-hour pump test. It is advisable to collect samples for pH, conductivity, bacteria, and turbidity prior to and during the sample collection to determine if the quality of the water is changing. If a surface water body is nearby, it would be advisable to monitor stream flow monitoring, collect a comparison composite sample from the stream, and collect a grab sample that should be tested for pH, conductivity, bacteria, and turbidity.

At least 300 gallons of water are passed through an AMF Cuno Micro Wynd II filter (DPPPY, 1 micron polypropylene core) mounted in a Carborundum Fulflo Filter Housing ( Model LT-10) collected over an 8 to 24 hour period. The inlet hose is connected to an appropriate tap and the pressure should not exceed 10 - 30 psi. Pressure over the filter face should be controlled by a pressure regulator and monitored with a pressure gauge. If the sample must be taken from a pump, it is recommended that the pump be situated downstream from the filter housing pulling water through it so that the pump and hoses will not introduce errors due to cross-contamination. If the sample is being collected from a spring box, the spring box should be clean prior to sampling and allowed to flush.

Prior to sampling, flush the main line to the filter for 15 minutes. Record time and meter reading before opening the sampling tap. Set flow rate to 0.5 - 1.0 gallons per minute depending on source water quality. After a sufficient sample is collected, turn off sampling tap, record time and meter reading, and disconnect sampling apparatus. Take care to maintain the opening on the inlet hose above the level of the opening on the outlet hose to prevent backwashing and to allow residual water to drain out of the filter canister.

With the bowl of the filter housing sealed, turn the unit upside down and allow excess water to drain into the Ziploc sampling bag from the filter housing. Open the filter housing and remove the filter; the filter should be handled with plastic disposable gloves and stored in a double Ziploc bag along with any residual water and rinsings from the filter housing. If the sample is chlorinated, it is advisable to add 50 ml of 1 % sodium thiosulfate to the bag and shake before bringing the sample back to the laboratory (recommended holding time is 48 hours). Since Giardia cysts are destroyed if they are allowed to dry or if they reach room temperature for any period of time, all samples should be transported on wet ice and stored in a refrigerator until they are processed. If blue ice is used, the filter must not be in direct contact with the blue ice packs because it could freeze. If the sample is received frozen, the filter can not be analyzed.

Risk Ranking for GWUI and MPA/ MET Evaluations For Surface Water Influence Testing

The following are the tables used to develop a risk ranging to determine the potential for surface water influence. The following tables are based on the number of specific bio-particles observed in the equivalent of 100 gallons of raw water. These tables are used to access springs and wells, but not filtration systems.

Table 1| Numerical Range of Each Bio-Indicator
Based on Numbers of Bio-particles Counted per 100 gallons of Water

Contaminant EH H M R NS
Giardia cyst > 30 16 to 30 6 to 15 1 to 5 0
Coccidia spore > 30 16 to 30 6 to 15 1 to 5 0
Diatoms > 150 41 to 149 11 to 40 6 to 10 0 - 5
other Algae > 300 96 to 299 21 to 95 6 to 20 0 - 5
Insects/ Larvae > 100 31 to 99 16 to 30 1 to 15 0
Rotifers/ Crustacea > 150 61 to 149 21 to 60 1 to 25 0
Plant Debris > 200 71 to 200 26 to 70 1 to 25 0

Count the particle if it contains chlorophyll

Source: PADEP "Surface Water Identification Procedure: Interpret the Microscopic Particulate Analysis, (WSCH1/93), 1993.

EH - Extremely Heavy ; H- Heavy; M - Medium; R- Rare; NS - not significant.

Table 2 | Relative Risk Factors Associated with Bio-Indicator Based on Numbers Counted per 100 gallons of Water.

Contaminant EH H M R NS
Giardia cyst 40 30 25 20 0
Coccidia spore 35 30 25 20 0
Diatoms 16 13 11 6 0
other Algae 14 12 9 4 0
Insects/ Larvae 9 7 5 3 0
Rotifers/ Crustacea 4 3 2 1 0
Plant Debris 3 2 1 0 0

Count the particle if it contains chlorophyll

Source: PADEP "Surface Water Identification Procedure: Interpret the Microscopic Particulate Analysis, (WSCH1/93), 1993.  

EH - Extremely Heavy ; H- Heavy; M - Medium; R- Rare; NS - not significant.

Risk of Surface Water Contamination:

Old Method (1992)

High ≥ 20
Moderate 19-Oct
Low < 10

New Method (1993)

Surface Water Indicators
Present ≥ 10
Absent < 10 

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