6/21/2011 SEWER COMMISSION Minutes

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Figure 2. Based on four test runs, the average infiltration rates in gpd/sf for Basin Number 1 was 972 gpd/sf; and based on five test runs, the average was 998 gpd/sf for Basin Number 2. The average result over both sites was 986 gpd/sf. Using the EPA criteria of 10 percent of the average infiltration test rate, a design loading rate of 98 gpd/sf is calculated. 3.4 Mounding Impacts Two observation wells were located in close proximity to the test basins. The water levels in these wells were monitored throughout the three days of testing at each location. Water level data is depicted graphically on Figure 3. There was minimal precipitation during the testing, which was initiated on April 18, 2011. Table 1 includes precipitation data for the testing period. However, the month of April saw higher than average rainfalls. Rainfall for the month of April totaled 5.40 inches, whereas the average rainfall for April is 4.25 inches (www.weather.com). Due to the excessive rain, the water tables may have been higher than normal. A rise of 2 to 2.5 feet in the water table elevation immediately adjacent to the test basins can be seen over the course of the three days of testing. The water level in the background well also rose approximately 0.5 feet over the course of the test, although this was not likely a result of the testing, as can be seen from the rising levels beginning prior to the start of the testing. A mounding analysis will be performed and the results will be incorporated into the WNMP. 4 TEST SUMMARY AND CONCLUSIONS Hydraulic loading tests were completed between April 18 and April 22, 2011. Testing was performed at two 10-foot basins. Testing was comprised of: 1. One day of saturating the soils beneath the basins for a 24-hour period by sustaining a water level in the basins equivalent to what was infiltrating out of them. 2. One day of constant head testing, during which water was fed to the basin at the same rate that water drained from the basin, as indicated by a constant water level in the basin. 3. One day of falling head testing, recording the rate at which the water level in the basins dropped. This was repeated six times. For the constant head testing, an average infiltration rate of 715 gpd/sf for the two basins was calculated. Applying the USEPA design factor of 10 percent to that result, an appropriate design loading rate is 72 gpd/sf. In the falling head testing, an average infiltration rate of 986 gpd/sf for the two basins was calculated. If this rate is used and the USEPA design factor of 10 percent is applied to that result, this would equal a design loading rate of 98 gpd/sf for the site. As part of completing their WNMP, the Town needs to estimate capacities for any potential future recharge facilities. The Town will be able to save significant amounts of money if it can avoid having to build redundant infiltration beds that would be based on an overly conservative design rate of 5 gpd/sf. Based on the USEPA guidance for acceptable loading rates for use in design, (10% of the observed rate) it is the recommendation that these findings be reviewed with MassDEP in order to establish an agreed upon loading rate for the site between the State valued of 5 gpd/sf and the observed 70 gpd/sf. GA86\12001\Hydraulic Load Tests\Tech Memo\DRAFT Site 4 Load Test 2011.DOC 6