GENERAL40527 - Laserfiche WebLink

Home Browse Search

l6- VZII. ALLUVIAL VALLEY FLOORS A.V.F. DETERMINATION The Seneca II permit and adjacent areas contain four areas with enough associated alluvium to be considered Alluvial Valley Floors (AVF); the confluence of Grassy Creek and Little Grassy Creek, a narrow strip along Little Grassy Creek, land adjacent to Fish Creek South of the permit area and a small fill located in the southern portion of the permit area. The latter, approximately IO acres, was determined by the Division to be too small to be an AVF. The confluence of Grassy Creek and Little Grassy Creek, the narrow strip along Little Crassy Creek and land adjacent to Fish Creek were determined by the Division to be Alluvial Valley Floors based on the geo- morphology, shallow ground water levels, soils, vegetation, land use and infra-red photos as demonstrated by the Following discussion. Exhibit 7-16 of the application delineates the areas that are characterized by alluvial fill material. There are no terraces associated with any of the areas. The confluence of Grassy Creek and Little Grassy Creek is approximately 95 acres with alluvial ma- terial averaging 90 feet deep. The strip along Little Grassy Creek is very narrow and comprises approximately 90 acres in total with alluvial material up to 51 feet deep. Grassy Creek and it's tributary, Little Crassy Creek, cover 3.07 square miles or 72~ of the permit area. The drainage from the mine area accounts for I2.Se of the overall Grassy Creek Basin. Two unnamed tributaries of the Fish Creek drainage basin are located in. the southern permit area. About 1.22 square miles or 1.50 of the total Fish Creek Basin will be affected by Seneca II. The quality and quantity of the two drainage basins is influenced strongly by spring runoff creating seasonal fluctuations as is characteristic of streams in the upper Colorado River Basin. The average flow in Grassy Creek, measured at SW-S2-2 (Exhibit 7-2) is 1,827 gpm with a recorded high of 22,280 gpm during spring runoff and a low of 28 gpm during low flow. During spring, alkalinity is lowest and during summer and winter alkalinity is somewhat higher. Acidity is the mirror image of alkalinity. The concentrations of total iron varies with suspended solids which increase during high runoff periods. Manganese exhibits the same seasonal fluctuations as iron and total suspended solids. pH has shown no apparent variations. Groundwater levels measured in alluvial wells GW-56-A, GW-531, GW-S33 (Exhibit 7-2) demonstrate subirrigation availability. The water quality analysis indicates a slight to highly mineralized Ca-Mg-SO9 or Ca-Mg-S04-HC03 type water with near normal pH and high sulfate concentrations. The amount of trace metals was low with the exception of high manganese levels in all wells. The application states that the high salinity of the groundwater makes it unsuitable for irrigation. The Division disagrees with this conclusion, since a comparison of water quality data collected during the growing season (Table 7-25) to a more recent flood irrigation water quality classification (US DI, 1968) shows a classification having a low salinity hazard. The soils found in the alluvial areas were derived from mixed, fine textured alluvium. They have a very slow permeability as a result of the high clay content which averages 400. The temperature class is cryic as is evidenced by the short growing season, ranging from 90-75 days. According to the S.C..S., this restricts the species that could be planted and most importantly the production. Due to the high clay content and the low production potential, the soils~do not lend themselves to crop production. They are best suited for grazing/rangelan d.