SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

Greg K. Vaughn • -5- . 11 May 1988 uppermost water-bearing zone o Extent On p. 3 .8-6 the HAR states that the near surface aquifer is 18 to 25 feet thick at the site and at nearby sites. No basis for this figure is cited, however, and no onsite borings have been advanced through the entire thickness of this aquifer. Deeper exploratory borings, as noted above, will identify the true thickness of the upper aquifer. o Gradients The water table gradient direction, as reported from monitoring over the period December 1985 through August 1987 , varies over a 1200 arc. The two downgradient wells, MW-1 and MW-2 are approximately southeast and south-southeast of the impoundment. At least two additional wells, one directly east and one directly south of the impoundment, are needed to accurately monitor downgradient water quality in the upper aquifer. Monthly monitoring results for the period from September 1987 through May 1988 should be included in the amended RAR to provide a better indication of the variability of ground water gradients. The magnitude of the water table gradient at the O/LDC site is very small during some months. Differences in water surface elevation between wells have been as low as 0. 03 feet. When this is combined with the variation in reference point elevations over many surveys at the site, the accuracy of gradient calculations comes into question. Additional wells at greater distances to obtain more reliable estimates of gradient direction and magnitude are needed. Water surface elevation measurements from the monitoring wells at nearby sites (cited on page 3 . 8-8) may serve the above purpose, but elevations must be tied to the O/LDC site survey. The seasonal ground water level information in Table 3 . 8 .4 should be presented in graphical form in addition to the tabular form. A ground water flow direction of SSE is reported (page 3 . 8-8) based on gradient directions for December 1985 through May 1987 . The presentation of mean flow direction in the HAR does not seem to consider the gradient magnitude, however. A weighted mean based on gradient magnitude and direction should be calculated to estimate a mean flow path. The gradient direction reported in the O/LDC August Ground Water Monitoring Report bears approximately N65E, nearly 900 from the mean direction. This variation is attributed to heavy pumping of domestic and irrigation wells nearby. Wells of sufficient capacity to affect water levels at the O/LDC site are more than likely not completed in the upper aquifer, but in deeper, more productive zones. If pumping of wells in deeper aquifers indeed affects gradients at the site, then the argument for lack_ of interconnection is weakened.