ARCHIVED REPORTS_XR0011036 - Laserfiche WebLink

Home Browse Search

from February to July 1991,and then rose 3 to 6 ft in August N after several weeks of rainy weather.These large changes in water-table elevation are presumably due to the low specific t U11 yield(high water retention)of the clay`confining unit.The average water-table gradient from March 1991 to October l NJ9 1992 was 0.0065 ft/ft. 2�-=-.� Short-term(15-minute), constant-rate, pumping tests o A were performed on wells screened in the sand aquifer(wells 0 01 1 EA A9,All,A18,A20,A21 in Figure 2).Theseetests were felt to "1 0 f be more accurate than slugtests without requiring the dispos- 0 %%- al of the large volumes of contaminated.water that result Ground �qPL from long-term aquifer tests.Hydraulic cou06ctivities were Water o Area calculated using Jacob's semilogarithn* approximation FI W p J (Domenico and Schwartz, 1990). First ti:��insrmssivity was A7 calculated by the"time-drawdown"method(Domenico and • Schwartz,1990);then,the hydraulic conductivity was found by dividing the transmissivity by the length of the screen Arvida Rd.— placed in.the sand aquifer.Results from tlwsbtests indicate the hydraulic conductivity of the sand aquifer:`.varies from 3 to 25 ft/d with a mean value of 10 ft/d.Basedoa'the average A•20 water table gradient and an effective poxosity of 0.25, the B B� ground-water velocity was estimated to ramie from 25 to 240 10 Al ft/yr with an average value of 100 ft/yr:: 7'.. Equilibrium retardation factors were:estimated using Estimated the measured soil organic carbon content:(0.001 mg/mg) Plume and an empirical relationship between the-soil partition Boundary A,912 coefficient(K x)and the octanol water partition coefficient (Kow)(Schwarzenbach and Westalf, 1981):log(Ko,)=0.49 +0.72 log(Kow).Estimated retardation factors range from 1.6 for benzene to 4.7 for m-,p-xylene. Drainage Ditch Methods A18 Residual Hydrocarbon Distribution In the area immediately beneath and downgradient from the former UST, nonaqueous phase liquid (NAPL) 600 ft hydrocarbon is trapped within the soil matrix. Three con- Fig. 2. Site map and monitoring well locations at the Rocky tinuous soil cores to a depth of 14 ft were collected in a Point site. Wells discussed in text are indicated by • symbol. longitudinal transect through the former tank pit to aid in Wells not discussed in text are shown by o.The plume boundary defining the horizontal and vertical distribution of residual was estimated to be the maximum extent of detectable BTEX. NAPL. These cores were collected by driving a 14n.- diameter conduit through the soil with an electric hammer. ' After advancing 2 ft,the conduit is jacked out of the ground; Hydrogeology sectioned,capped,and placed on ice for later analysis.This The relatively simple geology and geomorphology of technique was very effective and did not result in significant the Arvida site simplifies the hydrogeologic and contami- compaction of the soil. The BTEX concentration in each nant transport analyses. The fine-grained sand is the pri- sample was determined by adding a known mass of soil(—1 mary transmissive aquifer and pathway for contaminant gm)to I ml water,sonicating for 5 minutes,equilibrating at migration. The overlying clays and clayey sands form a 80°C,and measuring the BTEX concentration in the head- leaky, surface-confining layer throughout the site. In the space by gas chromatography. Volatilization losses should area north of Arvida Road,the piezometric surface varies be minimal since the soil samples were only exposed to the from 2 to 6 ft above the sand-clay interface depending on the atmosphere for a few seconds when they were transferred amount of recent precipitation.South of Arvida Road,the from the intact coring tube into vials in the lab. piezometric surface varies between 0 and 1.5 ft above this interface.Consequently,the aquifer is confined throughout Ground-Wafer Monitoring 15 JI the study area. Dissolved BTEX and related parameters were moni- The water table has a relatively gentle,constant slope in tored in ground water seven times over the period from July -" an almost due south direction.Ground-water levels in moni- 1991 to December 1992. Prior to sampling, all wells were toring wells changed significantly in response to recharge filled with prepurified argon gas to minimize. oxygen events. In the upgradient wells, water levels dropped 7 ft exchange with the sample and then purged of at least five 183