Dear Indra Vijitha, ...

Dear Indra Vijitha, Computation of Ground-Water Volumes Some calculations are available for estimation of ground water, but not foolproof procedure.Some techniques are developed in USA for finding ground water estimations,e.g.-sonic water level estimation,some eqations and formulae given for estimation of ground water level. The computation of volume of water in storage is illustrated with a hypothetical schematic of the basin attached fig-1. This schematic shows three of the six hydrogeologic units as nested bowls, with the bowl in the center of the basin, labeled as Tongue River-Wasatch aquifer, as the uppermost unit. The hydro geologic units increase in age with depth and outward from the centre of the basin. This schematic shows the water table not intersecting the land surface; however, areas may exist in the basin where this may happen. At any location, the saturated thickness for the Tongue River-Wasatch aquifer is given by a, which is the distance between the water table and the base of the Tongue River-Wasatch aquifer. The saturated thickness for the next lowest units, part of the Fort Union Formation, is more complicated. In areas where the potentiometric surface of Tullock aquifer is above the base of the Tongue River-Wasatch aquifer, the Tullock aquifer is confined and the saturated thickness of the Tullock aquifer, b, also is the apparent thickness of the Tullock aquifer. With increasing distance from the center of the basin, an area is reached where the water table is below the contact between the Tongue River-Wasatch aquifer and the Lebo Confining Unit (B). In this area (B), the Tongue River-Wasatch aquifer is completely unsaturated or absent. The amount of saturated thickness, c, is the difference between the water table and the base of the Tullock aquifer. The transition of aquifers between confined and unconfined conditions is poorly understood. On figure 5, the Tullock aquifer is confined at b. Applied Hydrology and Greystone Environmental Consultants, Inc. (2002, Appendix B) report that aquifer test results in alluvial sediments, which are part of the Tongue River-Wasatch aquifer as described in this report, indicate that at some depth even these materials act in a confined manner. In this study, an assumption was made that when a hydrogeologic unit has more than 50 feet of saturated thickness, it is confined. Thus, specific yield was used as the storage factor for a saturated thickness of 50 feet or less, and specific storage was used as the storage factor for a saturated thickness greater than 50 feet. Water-table data were from Hotchkiss and Levings (1986). The water table in the center of the basin (A in fig. 5) is from the approximate potentiometric surface in the Tongue River-Wasatch aquifer, whereas the water table (B in fig. 5) in stratigraphically lower units, represented by the Tullock aquifer in figure 5, is from potentiometric surface maps of other hydrogeologic units used in this report and given in Hotchkiss and Levings (1986). In some areas of the study, potentiometric contours of Hotchkiss and Levings (1986) did not extend to the edge of the outcrop. In these areas, water levels in shallow wells were obtained from the USGS Ground-Water Site Inventory database and a point coverage was made of these water levels. A TIN was assembled from this coverage. This was done only in areas where contoured water levels of Hotchkiss and Levings (1986) were not available. Any perched water-table zones were ignored in the calculation of ground-water volumes. The assumption was made that the extent of perched zones is small when compared to the entire area of the basin. For all volume estimates of saturated rock using the first method to estimate ground-water volume, the total volume of water was calculated by summing a component of water in sands and a component of water in non-sands. For the unconfined part of the aquifers, equation 1 was used to calculate the total amount of water in the sands: sand water = 1,000 * 1,000 * (sattk * ( % sand/100) * sand-porosity) (1) where: sandwater = the total amount of water in the sands, in cubic feet, 1,000 * 1,000 = the area of the grid cell, in square feet, sattk = the saturated thickness figure-1, in feet, %sand = the percentage of sand, and sand-porosity = the porosity of the sand. Equation 2 was used to calculate the total amount of water in the non-sand part of the cell: nonsandwater = 1,000 * 1,000 * (sattk * ( ( 100 - %sand) / 100) * non-sand porosity) (2) where: nonsandwater = the total amount of water in the non-sand component of the cell, in cubic feet, 1,000 * 1,000 = the area of the grid cell, in square feet, sattk = the saturated thickness (a in figure 5), in feet, %sand = the percentage of sand, and non-sand porosity = the porosity of the non-sand component. sfhvs = 1,000 * 1,000 * (fhtk * (fhsnd / 100) * Ssnd) * (trwsattk) (3) where: sfhvs = the volume of water released from confined storage in the sand part of the Fox Hills-Lower Hell Creek aquifer, in cubic feet, 1,000 * 1,000 = the area of the grid cell, in square feet, fhtk = the confined thickness of the Fox Hills-Lower Hell Creek aquifer, in feet, fhsnd = the percentage of sand in the Fox Hills-Lower Hell Creek aquifer, Ssnd = specific storage for sand, in feet-1, and trwsattk = the saturated thickness in the Tongue River-Wasatch aquifer, in feet. The procedure using equation 3 for the Fox Hills-Lower Hell Creek aquifer was repeated for each hydrogeologic unit. Estimated ground-water volumes were determined for the sand and non-sand parts of each unit using appropriate values of confined thickness, percentage sand, and specific storage.