20-06-2012, 12:22 PM
Bicarbonate
The value of HCO3- is observed from (0.9 to 58.9) which is the dominant ion except in the ground water occurring near the coastal area. Table1.
The lower concentration of Hco3- (320 and 340 Mg/L) is observed from the groundwater samples (S9 and S15) which contributes approximately 12% to 14% to the total anions while the higher concentration of Hco3- (460 to 590 Mg/L) is observed from the Remaining groundwater samples except (S9 and S15) approximately 92% which contributes 53% to 55% to the total anions Relatively low concentration of Hco3- (>400 Mg/l) observed from the low lying area where the marine clay occurs.
The computed values of pCo2 are varied from 10-2.2 to 10-2.4 atmosphere for all the groundwater samples following Raymahashay (1988). The values are significantly higher than that of atmosphere co2 (10-3.5 atm). Such elevated values suggests that the groundwater system is to open to soil Co2- Resulting from the decay of organic matter and root respiration, which in turn, combines with rain water to form Hco3-. The higher concentration of Hco3- in the water concentration of Hco3- in the water infers a dominance of mineral dissolution.
Calcium and magnesium:
The Concentration of Ca2+ is between (84.78 and 200 Mg/l). While that of the concentration of Mg2+ is varied from (35.63 to 164.4 Mg/l) Table 3 Relatively the higher concentration of Mg2+ is observed between (148.8 and 164.4 Mg/l) from the ground water samples (S2 and S16) . The rest of the groundwater samples show Mg2+ from (35.63 to 106.8). The contribution of ca2+& Mg2+ to the total cations is approximately 7% to 19% and 13% respectively. The higher contribution of Mg2+ than that of the contribution of Mg2+ than that of the contribution of Ca2+ is caused by the influences of ferro magnesium minerals, ion exchange between Na+ and ca2+, precipitation of CaCo3, and marine environment.
Sulphate:
The value of So42- is observed from 21to145Mg/L (Table3).The highest So42- content of 130 to 145Mg/L in the groundwater samples29 and 30indicates the influence of sarine source. The application of gypsum (CaSo4, H20), as amendment to alter the physical and chemical properties of the soils, is expected to be caused for the concentration of So42- in the range of 21 to 93Mg/L in the study area, in a general case, since no lithiological units occur. So, the contribution of So42- to the total cations is approximately 7% to 8% near the coast while it is approximately 3% to 6% in the rest of the area.
Nitrate:
The value of No3- in the ground water is observed between 19 and 53 Mg/L. (Table 3). The groundwater shows a very low content of No3- from 19 to 21 Mg/L near the coast, which is approximately less than 1% of the total anions compared with the other well water that has No3- between 26 and 53 Mg/L, which contributes approximately 2% to 5% to the total anions. The No3- is a non-lithological source, In natural conditions, the concentration of No3- does not exceed 10mg/l in the water (cushing et al. 1973) so that the higher concentration of No3-, beyond the 10 mg/L is an indication of No3- anthropogenic pollution. It is mainly due to influences of poor sanitary conditions and indiscriminate use of higher fertilizers for higher crop yields in the study area.
Sodium and potassium:
The concentration of Na+ is varied from 192 to 453 mg/L (Table 3) in a general case which is the dominant ion among the cations contributing approximately 53% to 69% to the total cations. This is because of the silicate weathering and/or dissolution of soil salts stored by the influences of evaporation and anthropogenic activities (stallard and Edmond 1983; Meyback 1987; subba Rao 2002), in addition to the agricultural activities and poor drainage conditions. Moreover the solubility of Na+- salts is generally high. The higher concentration of Na+ from 588 to 648 Mg/L is observed from the groundwater samples 29 and 30.
In contrast of the concentrations of Ca2+, Mg2+of Na+ ions among the concentration of K+ is observed between 10 and 38 Mg/l from the ground water (Table3) because the potash feldspars are more resistant to chemical weathering and is fixed on clay products.
Sodium adsorption ratio:
Excess sodium in waters produces the undesirable effects of changing soil properties and reducing soil permeability (Kelly 1951). Hence the assessment of sodium concentration is necessary while considering the suitability for irrigation. The degree to which irrigation water tends to enter into cation-exchange reactions in soils can be indicated by the sodium adsorption ratio (U.S. Salinity Laboratory 1954). Sodium replacing adsorbed calcium and magnesium is a hazard (Todd.1980). The waters were classified in relation to irrigation based on the ranges of SAR values (Richards 1954).
SAR= Na+/ (√ca2++mg2+/2)
According to Richard’s classification all the samples of the study area have been classified as excellent for irrigation. SAR values of the water samples in the virgin area vary from 0.38 to 1.67 and in the mine area from 1.20 to 2.18.
Residual sodium carbonate:
In addition to total dissolved solids, the relative abundance of sodium with respect of sodium with respect to alkaline earths and boron, and the quantity of bicarbonate and carbonate in excess of alkaline earths also influence the suitability of water irrigation. This excess is denoted by ‘Residual sodium carbonate’ (RSC) and is determined as suggested by Richards (1954). The water with high pH and land irrigated by such waters becomes infertile owing to deposition of sodium carbonate as known from the black colour of the soil (Eaton, 1950).
According to the U.S. Salinity Laboratory (1954), an RSC value less than 1.25 Meq/l is safe for irrigation, a value between 1.25 and 2.5 Meq/l is unsuitable for irrigation. In the present study the waters in both virgin areas and the mine show RSC Values of -300.2 to 10.9 Meq/l which includes both the safe and marginal quality categories for irrigation. Further, continued usage of high residual sodium carbonate waters affects crop yields.
PERMEABILITY INDEX:
The soil permeability is affected by long term use of irrigation water. Sodium, calcium, magnesium and bicarbonate content of the soil influence it. Doneen (1964) evolved a criterion for assessing the suitability of water for irrigation based on a permeability index (P.I) where P.I = Na+ √Hco3/(Ca+ Mg +Na) *100.
According waters can be classified as Class I, Class II, and Class III orders. Class I and Class II waters are categorized as good for irrigation with 75% or more of maximum permeability. Class III waters are unsuitable with 25% of maximum permeability. Accordingly all the samples fall into the class I category of Doneen’s chart. In the present study, P.I. values vary from 29.29 to 48.40 in the mining area samples and from 26 to 61.80 in the virgin area samples. These values are more pronounced in the virgin area than in the mining area samples.
Kelly’s ratio:
Based on Kelly’s ratio waters are classified for irrigation. Sodium measured against calcium and magnesium was considered by Kelly (1940) and paliwal (1967) to calculate this parameter. A Kelly’s ratio of more than one indicates an excess level of sodium in waters. Therefore, waters with a Kelly’s ratio of more than one are suitable for irrigation, while those with a ratio more than one are unsuitable. Kelly’s ratio in the present study varies from 0.06 to 0.49 in the virgin area and from 0.25 to 0.55 in the mining area samples. Therefore, according to Kelly’s ratio, all of the water samples are suitable for irrigation.
Magnesium Ratio:
Generally calcium and magnesium maintain a state of equilibrium in most waters. In equilibrium more Mg in waters will adversely affect crop yields. As the rocks of the study area consist of dolomites, it is observed that most waters contain more Mg than Ca. In the present study 98% of samples contain a Mg ratio more than 50%. This would adversely affect the crop yield as the soils become more alkaline. In the present study, the ‘Magnesium ratio’ values vary from 14.99 to 95.81 in the mining area samples and from 61.74 to 93.66 in the virgin area samples. These high values are because of the presence of dolomite in these areas.