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Effects of Urbanisation on Base Flow and Ground Water
1)Pinder GF, Jones JF, 1969
Components of ground water and stream discharge are determined from the characteristics of stream water. The presence of a chemical mass balance determines the relationship that lies between the direct and ground water runoff. Knowing the chemical compositions of ground water and the direct runoff water is essential. The total runoff resolves the solute concentration of ground water and samples of total runoff explain the chemical concentration. The inability to determine the actual ground water discharge is due to the high water and floods.
Earlier, graphical separation of the discharge hydrograph by exploration of ground-water recession curves beneath t he flood peak. There are different types of graphical representation of the analysis one obtains. These differences vary from the very simple methods of carrying out the analysis to the complex methods. When studying the relationship between the stream chemistry and the discharge, it is vital to select basins where precise and continuous discharge hydrographs are essential. Surfficial deposits that occur at the Long Island are due to glaciations. This process takes place through the dumping of glacial debris within the shores with sloping glacio-fluvial sand and gravel. Swamp and mash deposits also form lines along the shore.
Within the shore, the ground water contains four aquifers. The lower three are confined while the upper glacial aquifer connects hydraulically to the streams and lake. The water table aquifer is the origin of base flow to all streams and lakes even though it is no longer extensively developed for public water supply. The south flowing streams in the long island are widely spaced and have few tributaries with gentle gradients. The mean annual precipitation occurring in the area ranges from slightly less than 41 inches to 50 inches. The high permeability of the glacial outwash sand and gravel and also a flat terrain gives room for precipitation to infiltrate immediately.
The concentration of ions in the total runoff is as a result of the concentration of ions in ground water and direct runoff. Assuming the absence of chemical interactions implies that the direct runoff includes subsurface flow in direct runoff. This brings into picture the method used in estimating the chemical composition of direct runoff, which entails analyzing the samples collected. Another method used in determining the chemical composition of direct runoff involves obtaining samples from the main channel during peak flow. In basins with extensive flood plains, deposits and ground water discharge as vertical seepage into the main channel. During this period, water flows from the river into the adjacent fluvial deposits making ground water unable to discharge vertically into the bottom of the channel.
In basins that face this type of hydrologic system, the samples obtained from the peak discharge provide an estimate of the chemical characteristics of the direct-runoff component. It is vital to also note that an increase in solute concentration may be due to the ground water containing high concentrations of ions in the lower segment of the basin. This means that a decrease in the ground water discharge is as a result of a lower hydraulic gradient towards the main channel. In short, this observation shows that ground water discharges continues during peak periods of peak flow. The chemical characteristics collected are also similar in upstream areas, and the samples gathered from the upper most station during periods of peak discharge provide a good estimate of the chemical characteristics of direct runoff.
The higher concentration of precipitated ions is as a result of limestone and gypsum underlying the basin. Ions that are more concentrated in direct runoff than ground water include nitrate and iron. Decaying organic matter is the primary source of nitrates in the basins. This is because fertilizer is not widely used in these areas. The solution of oxidized iron sulphide minerals that are present in the glacial till account for high iron and sulphate concentrations in the runoff.
This article, however, do not put into account the effects of urbanisation and use of land on ground water flow and penetration. Increase in urban areas, industrial and commercial premises increases storm runoffs resulting to loss of ground water recharge. This in return affects the penetration of ground water.
2Simmons DL, Reynolds RJ, 1982
Currently there are numerous techniques coming up to determine the ground-water components of the total runoff. Before urbanization, 95% of the total ground water flow occurred abs base flow. However, urbanization along the coastal shores has resulted into the considerable loss of ground water flowing into the streams.
The increase in population of the inhabitants of Nassau and Suffolk has doubled from what it was in 1950. The increase in population especially in the 1960s has greatly enhanced development near the shores. The land use patterns also affect the water flow. In Suffolk, 15% of the area was covered by houses and streets, 10% for commercial and industrial purposes while 75%was left open for agriculture. Urbanization in Nassau gave room for the permeable land increasing the volume of urban storm runoff and loss of ground water recharge. To cub this problem, there was construction of recharge basins to contain the storm runoff and replenish the upper glacial aquifer.
The waste disposal system also played a great role in determining the ground-water runoff. Earlier on, septic tanks and cesspools were used. Despite being effective in conserving ground water, they introduced contaminants into the soils of the upper glacial aquifer especially nitrates. To avoid further occurrence of the damage, there was the development of a sewer system that ensured all the wastes to treatment centres before being discharged into tidewater.
In conclusion, it is vital to note that the water base flow is essential and effective measure ought to be in pace to main these levels of ground water. Discharges into the water affect the final chemical composition of this water. The sewage also ought to be well treated to avoid contaminating the water. The percentage of chemical substances present in the water is in relation to the activities that take place next to the water levels.
This article, however, ignores the fact that the ground water and stream discharge is affected by many other factors and not just the effects of urbanisation on the streams. These factors include the characteristics of soil around the stream and chemical characteristics of the water in the stream. These determine to a great extent, how water penetrate through the ground.