Buried Bedrock Valleys Filled with Glacial Deposits - Essay Example

appears here] appears here] appears here] appears here] Buried Bedrock Valleys Filled with Glacial Deposits Ground Water Occurrence and Nature of Flow When precipitation falls (Figure 1) it may: (i) be evaporated from the earth's surface or from the leaves of plants (evapotranspiration) whose roots have taken up the moisture from the soil; (ii) flow along or near the surface of the earth in watercourses of ever-increasing size until it reaches the ocean; (iii) infiltrate down through the pores or crevices of the earth's mantle either at the point where it falls or at some distant point to which surface flow has carried it. Water which evaporates from the earth's surface or bodies of water is ready to start the cycle over again as precipitation. When water is added to dry or unsaturated soil it is held in the voids between particles by capillary forces. Once the voids are saturated, however, the water is free to descend under the effect of gravity. As long as there is sufficient water to maintain saturation, the water will descend until it is stopped by some impervious layer, such as rock or highly impervious clay. The water can then flow laterally through the voids or rock crevices above the barrier. If there are significant differences in surface elevation, the water may flow out along the impervious layer at some lower point called a spring. If a hole is made vertically down into the saturated layer, water will flow into the hole. If the saturated layer has sufficient interconnected voids, water will flow through it relatively rapidly. When the saturated layer yields water in economic quantities, it is called an aquifer and the hole made into it could be developed into a well. The lack of resistance to flow through porous material is called permeability. In general, fine grained material such as clay or silt is low in permeability; sand is of medium permeability, and gravel is most permeable. Fractured rock varies in permeability depending on the degree and pattern of fracture. The quantity of water which can be stored in an aquifer is equal to the total volume of voids between the solid particles. The fraction of the total volume of an aquifer made up of voids is called porosity. If the voids are interconnected, aquifers of high porosity also tend to have high permeability. Sometimes groundwater is trapped under an impervious layer. An aquifer thus located is called a confined aquifer. If the inflow area to a confined aquifer is higher than the confining layer where a well penetrates it, the water will be under pressure and will rise in the well to some level above the confining layer. Such a well is referred to as artesian. If the water rises to the top of the well a "flowing well" results. Obviously some locations offer better chances for successful wells than others. Clues which can be helpful in selecting well locations are (i) locations and depth to water of existing wells; (ii) existence of springs and/or streams; (iii) relative locations of infiltration areas and rock outcroppings which might constitute an impervious layer; and (iv) existence of known phreatophytes (plants requiring abundant water, whose roots frequently extend to the water table). In some areas of uniform geology, such as certain alluvial deposits in valleys, wells can be constructed anywhere with equal success. In the absence of any clues or data, a test boring can be carried out by one of the methods described under small diameter wells. Such a boring can be carried out relatively quickly and cheaply and can save considerable time, money and frustration in the long run. When a well is pumped, the water in it drops to some level below the static level (Figure 2). The water surface in the aquifer then forms a "cone of depression" as it slopes from the static level at some fairly large radius, R, to the well whose radius is r. If the well completely penetrates the aquifer with the static height of water being H and the height of water during pumping, h, then theoretical considerations give: Where: Q = yield or rate of pumping (e.g. m3/hr, litres/sec, etc.) K = permeability of the aquifer (H - h) is known as the "drawdown" of the well. If the drawdown is small compared to H, then the term (H + h) is approximately equal to 2H and the yield, Q, is approximately proportional to the product of H times the drawdown. This shows the importance of penetrating the aquifer to an adequate depth. By contrast the yield, Q, is much less responsive to changes in well diameter, since it is inversely proportional to the natural log of the ratio, (R/r). (http://www.fao.org/documents/show_cdr.aspurl_file=/docrep/X5567E/x5567e03.htm) Buried Bedrock Valleys All the way through the Blanchard-Oldtown area, crystalline bedrock comes about at or close to the surface in the surrounding highlands. The designs of its surface below the prevailing lowlands have been inferred from geologic maps of the region as well as well log data. The outline of the buried bedrock actually looks like a narrow trough or valley. The area of the bedrock valley is mainly described by the contact between the alluvium and bedrock particularly along the margins of Hoodoo Mountain to the east and the neighboring mountains to the southwest and west. The northwestern boundary of the valley is shaped by bedrock high whose southern area is exposed in Township 55 North, Range 6 West and in Washington. Right away south of this area, a small tributary valley trending northwest-southeast crops up and has its convergence with the main valley in the vicinity of Tweedie, Washington. The common slope of the bedrock surface emerges to be somewhat precipitous and abrupt towards the axis of the valley. Though, two famous divergences from this take place where buried bedrock ridges broaden to the west off of Hoodoo Mountain. The larger of the two lies just south of the Pend Oreille River and contains a subsurface expansion of a ridge exposed in Township 56 North, Range 5 West. The further buried ridge, whose axis approximately concurs with the border of Township 55 North, Range 5 West, has a noticeably more subdued surface expression. (Walls, Timothy A., MALONE, David H., and NELSON, Robert S, 2004). In accordance with the studies on the topic of the drainage history of the region, there is strong proof that the Pend Oreille Valley under Old town was not at all times occupied by a northward-flowing stream. The wide thickheaded angles of the tributaries incoming the Pend Oreille River and the thin youthful valley near Metaline Falls, Washington at its obvious familial headwaters, all propose a previous drainage to the south. Depletion from the ancestral stream was considered to have been through the Little Spokane Valley, though, founded on a comparison of the altitudes of the bedrock surface; drainage to the south towards Blanchard for as a minimum part of its drainage history is more probable Southeast of Blanchard, the stream in fact merged with a main ancestral drainage system in Rathdrum Prairie. Throughout the Pleistocene i.e., two million to ten thousand years ago, glacial activity considerably customized the profile of the valley and was accountable for the majority of the sediment buildup in the area. Scouring of the bedrock cropped up as glacial ice highly developed into the region from the north and sourced the valley to be broadened as well as deepened, mainly in the northern part. Glacial till, which is made of a heterogeneous mixture of rock debris and sediment, was deposited in a straight line by the ice in the northern part of the valley. Melt water from the ice deposited sands as well as gravels in the southern division of the valley. By retreat the glacial ice, coarse outwash built up on top of the till in the northern part of the valley. The width of the valley fill goes above three hundred feet in the surrounding area of Old town and is as a minimum two hundred feet near Blanchard. Groundwater Occurrence and Nature of Flow in Buried Bedrock Valleys Ground water crops up in both the bedrock as well as overlying sediments; though, their capability to stock up and pass on water differs significantly. Crack within the bedrock present the merely roads for ground water to move through. Even though a few fracture zones perhaps tremendously transmissive, their size and extent are normally limited and consequently so is their storage capability. Wells completed in the bedrock have accounted discharges of less than ten gallons per minute. The water bearing properties of the sediment are directed by the amount of interconnection between the individual minute opening spaces. Because these openings are bigger in the coarser grained deposits, sands as well as gravels can stock up and transmit significantly greater quantities of water than silts and clays. Production data from wells completed in the sediments point out that particular capacity vary from less than one to almost ten gallons per minute per foot of drawdown for fine sands and silts, to bigger than seventy-five gallons per minute per foot for coarse sands and gravels. By making use of a particular value for specific capacity, an estimated amount of drawdown in a well can be predicted for a known pumping rate. for instance, if a hundred gallons per minute per foot rate is wanted from a water-bearing material composed of fine sand and having an expected specific capacity of five gallons per minute per foot then around twenty feet of drawdown would be needed i.e., hundred gallons per minute divided by five gallons per minute per foot. (Cole, J.; Coniglio, M.; and Gautrey, S, 2005) The source of all water that comes into the hydrologic system is from rain as well as snow fall. Mean yearly precipitation varies from about twenty five inches in the lower elevations to more than forty inches in the higher elevations. Around sixty percent of the precipitation falls as snow throughout the winter months. Since fundamentally no surface water leaves the drainage basin, the entire unconsumed water ultimately turns into ground water. This can take place all the way through direct infiltration of precipitation into the cracked bedrock in the highlands or permeable sediment in the lowlands. On the other hand, the majority runoff is most likely concentrated at the headwaters of the various drainages due to the high proportion of impervious bedrock. While the runoff enters the lowlands, it possibly will quickly infiltrate the porous soils present, or be for the time being retained in lakes and sloughs are common in the area south of Tweedie, Washington. A comprehensive water-table map was made of the Blanchard- Oldtown area from water-level dimensions completed during a field visit in 1987. Water levels reported by drillers were used like a general help where measured data was missing. The configuration of the buried bedrock surface as well helped in the map construction. The contour of the water table reveals a subdued duplication of the buried bedrock surface. By the side of the margins of the valley, the slope of the water table is precipitous as well as indicative of the limited flow conditions within the fractured bedrock. The two buried bedrock ridges illustrate an alike, however to some extent reduced consequence on the flow system. As ground water runs into the porous sediments that fill the valley, the hydraulic gradient suddenly evens out. In the surrounding area of Township 55 North, Range 5 West an obvious ground-water divides crops up. North of the divide, ground water flows northwesterly towards Old town and ultimately issues as springs as well as seeps in the Pend Oreille Valley. South of the divide, ground water flows southwesterly and in fact combines with underflow leaving Spring Valley as well as the tributary bedrock valley in Washington. This combined flow carries on southward towards Blanchard, and after that beyond to eventually join the chief flow system in Rathdrum Prairie. A balanced water body is considered to be present in the northern part of the area. Due to the presence of glacial till of little permeability below an altitude of about two thousand two hundred and fifty, ground water is for the time being retained in the overlying coarse outwash, generating a perched aquifer. Water not removed by discharged into the Pend Oreille Valley, ultimately arrives at the main water table by means of vertical leakage. Wells completed in the suspended aquifer are for domestic uses and have accounted discharges of ten gallons per minute or less. Significant water-level trends in the area are fundamentally unknown. Three of the wells visited were calculated by the U.S. Geological Survey (USGS) but present very restricted information. Wells 54N-5W-18AAA1 as well as 56N-6W-36AAA1 both explain net declines over the 10 year period. The declines vary from more than four feet to almost nine feet, correspondingly. The third well, 55N-6W-lDl points out a net increase of greater than twelve feet for the 10 year period. A number of conditions could have an effect on the obvious rise or fall of the water table in these areas, on the other hand, changes in the quantity of ground water that is withdrawn and differences in the amount of water that yearly enters the hydrologic system almost certainly have the maximum areal impact. (Kelly L. Warner, 2002) Fig. 1 Hydrologic cycle Fig. 2 Flow into a well Reference: Cole, J.; Coniglio, M.; and Gautrey, S, 2005. Regional distribution and development of porosity in karstic aquifers in the Guelph area of southern Ontario: Implications for groundwater resources Kelly L. Warner, 2002. Arsenic in Glacial Drift Aquifers and the Implication for Drinking Water-Lower Illinois River Basin; Journal of Ground Water - Abstracts, Volume 39, Number 4 Walls, Timothy A., MALONE, David H., and NELSON, Robert S, 2004. GEOPHYSICAL INVESTIGATION OF THE TICONA BURIED BEDROCK VALLEY, MARSEILLES QUADRANGLE, LASALLE COUNTY, ILLINOIS; Department of Geography-Geology, Illinois State Univ http://www.fao.org/documents/show_cdr.aspurl_file=/docrep/X5567E/x5567e03.htm Read More