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<br />~ <br /> <br />TV 0 <br />1^-L. <br /> <br />Chemical Water Quality <br />The chemical quality of groundwater as measured by dissolved solids <br />concentrations is less than 500 mg/l from Chatfield Reservoir down to <br />Columbine Valley. From Chatfield Reservoir to Brighton the groundwater in <br />the alluvium of the South Platte River has a dissolved solids concentration <br />in excess of 500 mg/l. <br /> <br />Well Yields <br />Well yields from the South Platte alluvial aquifer are satisfactory for the <br />entire 40 mile study reach. Individual pumping rates of from 500 to 1..000 <br />gpm are common. There are at least 15 zones where the well yields range <br />between 1,000 and 1,500 gpm, with some wells pumping in excess of 1,5DO <br />gpm. <br /> <br />Figure IX-l presents the zones of various depth to water table. Also shown <br />on Figure IX-l are the results of water ievel medsurements in two selected <br />wells near Brighton between 1955 and 1977, <br /> <br />Well Pumping Effect on River <br />The alluvium is hydraulically connected to the surface stream of the South <br />Platte River. Pumping of water from the wells causes a depletion to the <br />surface flow, but the depletion is lagged over time. <br /> <br />More detailed analyses which have been performed for speCific water supply <br />investigations indicate that, commencing at about 1-70 the dissolved solids <br />concentration is approximately 1,000 mg/l. Some variations in dissolved <br />sol ids concentration is found laterally within the aquifer, the better water <br />quality existing in some places near the west edge of the alluvium. <br /> <br />Pumping for one month, for instance, may cause a depletion to the river of <br />ten to twenty percent of the amount pumped during the first month. The <br />balance of the total depletion may cause an additional six to twelve months <br />of river depletion while the drawdown aquifer is refilled, i.e., refilled by <br />direct recharge from the river or from interception of lateral recharge from <br />the adjacent lands. <br /> <br />Pollutants from man's activities, such as storm runoff and sewage effluent <br />discharges, adversely effects the groundwater quality. For instance, Brigh- <br />ton has experienced nitrate problems in its municipal water supply. Sul- <br />phate levels are also high in the downstream portion of the aquifer. <br /> <br />Gravel Mining <br />When an acre foot of gravel is removed from below the water table of the <br />alluvium of the South Platte River in the process of gravel mining, it is <br />replaced with water. <br /> <br />The City of Thornton experienced high levels of nitrite in the late 1970's <br />from their well field area upstream of 88th Avenue. <br /> <br />Prior to mining, an acre foot of gravel would have contained approximately <br />0.25 acre-feet of water as saturated aquifer. By removing the gravel the <br />volume of water within the same space is increased from 0.25 to 1.0 acre- <br />feet. Thus, with mining, the water in the mined floodplain zone is <br />increased. <br /> <br />Protection of the aquifer water quality is necessary. Leachate from solid <br />waste landfill sites and millsite waste piles adds various dissolved con- <br />stituents, Return flow from irrigation adds a variety of salts to the <br />groundwater. An increase in total dissolved solids in the groundwater is a <br />normal result of irrigation in Colorado. <br /> <br />Within the aquifer, the water table slope is both towards the river and <br />downstream. The downstream component of the slope is generally parallel to <br />the slope of the river. <br /> <br />Gr av e 1 <br />slope. <br /> <br />mining, where an open pit is left, disrupts the normal groundwater <br />The gravel pit lake is flat. The water level in the upstream <br />