EXPLORATION OF PHYSICO-CHEMICAL PARAMETERS AND IONIC CONSTITUENTS FROM GROUNDWATER USED IN IRRIGATION OF TANGAIL DISTRICT , BANGLADESH

Physico-chemical parameters, Groundwater, Irrigation, Tangail district The study was conducted for exploration of physico-chemical parameters and ionic constituents of groundwater used in irrigation of four upazilas namely Tangail Sadar, Kalihati, Delduar and Nagarpur upazila of Tangail district during the months of March, April and May of 2015. The physico-chemical parameters (pH, EC and TDS), ionic constituents (Ca, Mg, Na, K, Cl, CO3, HCO3, PO4 and SO4) and trace metal (Fe and Mn) were analyzed to assess the quality of irrigation in relation to soil properties and crop growth. The pH of groundwater indicates slightly alkaline in nature. As regards to EC the groundwater was in ‘good’ class and medium salinity hazards in quality for irrigation and the concentration of TDS indicates water as fresh water. The concentration of Ca, Mg, Na, K, Cl, CO3, HCO3, PO4 and SO4 of groundwater were recorded within the permissible limit for irrigation and these ions might not create hazardous impact on soil ecosystem for growing crops. The trace amount of Fe and Mn was detected in irrigation water. In the study area, the groundwater was within the recommended limit and would not create problem for irrigation and that have not long term effects on irrigating agricultural crops which could be safely used for irrigation purposes.


INTRODUCTION
Groundwater is an important source of fresh water for irrigation use in many regions of Bangladesh and is a component of the nation's freshwater resources.About 75% of cultivated land is irrigated by groundwater and the remaining 25% by surface water.Groundwater about 70-90% is used for agricultural purposes and the rest for drinking and other water supplies (BBS, 2006).The chemical quality of groundwater is considered as an important criterion for long-term irrigation because of containing the relatively high content of various ions as dissolved chemical constituents as compared to surface water (Ayers and Westcot, 1985).All water contain varying amount of different types of ion (cations and anions).Among them the main soluble constituents are Ca 2+ , Mg 2+, Na + and K + as cations and Cl -, SO4 2-, PO4 3-, CO3 2-and HCO3 -as anions.Out of the soluble constituents, Ca 2+ , Mg 2+ , Na + , Cl -, SO4 2-, and HCO3 -are of prime importance in judging the water quality for irrigation (Golterman, 1971).If the polluted groundwater is applied for irrigation, some ions may accumulate in soils as well as crops and deteriorates soil environment ultimately affecting crop production (Ayers and Westcot, 1985).The quantity and quality of different ions in groundwater and surface water system influences its better utilization for irrigation.Nowadays, agrochemicals are applied indiscriminately under intensive agriculture and as a result, lead to ionic contamination of groundwater and surface water (Lal and Stewart, 1994;Schwartz and Zhang, 2012).Recently, the increased attention has been paid in water for monitoring and management of these ions for using safe water.
Tangail region groundwater is mainly applied for irrigation.During the dry season farmers are completely depended on groundwater for irrigation.The cropping sequences like HYV rice, vegetables and rabi crops are found to be cultivated under irrigation.For this purpose, the quality of irrigation water must be evaluated and to identify the concentrations of ionic compounds that are important for plant growth.Keeping the above fact in judgment, the study was conducted to explore physico-chemical parameters and ionic constituents from groundwater and to evaluate the suitability of water with irrigation standard for crop production.

Study area
The study area was located at the four upazilas namely Tangail Sadar, Kalihati, Delduar and Nagarpur upazila of Tangail district (Figure 1) that occupies an area of 33426, 29560, 18454 and 26270 acres respectively.The total land areas of Tangail district are 835422 acre in which cultivable area 577701 acre and irrigated area 285914 acre (BBS, 2012).

Sample collection
Groundwater samples were collected during the months of March, April and May (2015) from four upazilas of Tangail district.The each of four upazilas were divided into threes sampling points and from each points twelves samples were collected from each of three months (March, April and May) and finally total thirty six groundwater samples were collected from the agricultural sites of the study area.Groundwater samples were collected from 12 shallow tubewells.At the time of sampling, all groundwater samples were free from colour, odour and unpleasant taste.All samples were filtered through filter paper to remove undesirable solids and suspended materials before chemical analysis.The collected groundwater samples were tightly sealed as quickly as possible to avoid air exposure and analyzed immediately.

Sample Analysis
The pH, EC (Electrical conductivity) and TDS (Total Dissolved Solids) of groundwater were measured by pH, EC and TDS meter, respectively (APHA, 2012).The concentrations of Ca 2+ and Mg 2+ in water samples were estimated by EDTA tritimetric method (Page et al., 1982) whereas K + and Na + contents were determined by flame photometric method (Ghosh et al., 1983).The concentrations of Cl -, CO3 -and HCO3 -in water samples were analyzed by titrimetric method (Tandon, 1995).The content of PO4 3-and SO4 2-in water samples were determined by spectrophotometric method (Tandon, 1995) where as the trace metal Fe and Mn were analyzed by atomic absorption spectrophotometric method (APHA, 2012).

RESULTS AND DISCUSSION
The pH of all groundwater samples were ranged from 7.53±0.05to 8.07±0.06 which indicating slightly alkaline in nature and was suitable for crop production (Table 1) where as the acceptable limit of pH in groundwater for irrigation is from 6.50 to 8.40 (Ayers and Westcot, 1985).The higher value of pH represent that there is high chloride, carbonate, bicarbonate etc. (Michael, 1978).
As regards to electrical conductivity (EC) of all groundwater samples was found within the limit of 373.67±34.64 to 463.33±20.21µScm -1 (Table 1).All groundwater samples were considered as medium salinity hazard (C2, EC= 250-750 µScm -1 ) and could be safely used for crops growing on soils with moderate level of permeability and leaching as mentioned by Richards (1968).The measured total dissolved solids (TDS) varied from 210.00±5.57to 288.33±7.64ppm (Table 1).All groundwater samples were considered as fresh water (TDS < 1000 mg L -1 ) in quality (Freeze and Cherry, 1979) and that could be used for irrigation purposes.
The concentration of Ca 2+ of all groundwater samples were ranged from 34.01±1.50 to 63.38±3.58mg L -1 (Table 2).Irrigation water containing less than 100 mg L -1 of Ca content is "suitable" for raising crop plants, as mentioned by Todd (1980).On the basis of this acceptable limit, Ca 2+ status of all collected groundwater was not treated as contaminants for irrigation purpose.According to Ayers and Westcot (1985), irrigation water containing below 121.5 mg L -1 (20 me L -1 ) of Mg 2+ is suitable for irrigating crops.The measured Mg 2+ content of groundwater samples were varied from 21.52±0.30to 40.04±0.46mg L -1 (Table 2) that was within the acceptable limit and could safely be used for irrigation.In groundwater samples, the status of Na + , K + and Cl -ions ranged from 19.26±0.31 to 26.10±0.78mg L -1 ; 0.63±0.06 to 0.92±0.06mg L -1 and 0.57±0.07 to 0.88±0.03me L -1 , respectively (Table 2).On the basis of Ayers and Westcot (1985), the accepted usual limits of Na + , K + and Cl -ions are 920.00mg L -1 , 2.00 mg L -1 and 4.00 me L -1 respectively.Considering these limits, all groundwater samples containing these ions had no remarkable impact on soil properties and crop growth when applied to soil as irrigation water.
The concentration of CO3 2-did not get in all the groundwater samples.So the pollution of irrigation water by this ion dose not arises at all.Bohn et al. (1985), as reported that the concentration of CO3 2-should be negligible at pH < 9.0 which might be the reason of finding the samples carbonate free, as because the pH of all groundwater samples varied from 7.53 to 8.07.The concentration of HCO3 -of all groundwater samples were ranged from 0.64±0.09to 1.17±0.06me L -1 (Table 2) where as the recommended maximum limit of HCO3 -for irrigation water used continuously on soil is 1.50 me L -1 , as mentioned by Evangelou (1998).So, the groundwater of study area was suitable for irrigation on the basis of HCO3 -content and has no possibility of soil hazard.
The concentration of PO4 3-ion in all groundwater samples were measured from 0.15±0.01 to 0.22±0.02mg L -1 (Table 2) and was within the recommended limit (2.0 mg L -1 ) as reported by Ayers and Westcot (1985).The groundwater is not hazardous for long-term irrigation and showing no impact on soil properties and crop growth.In all groundwater samples, the SO4 2-content ranged from 2.14±0.05 to 2.73±0.05mg L -1 (Table 2) and was not problematic when applied to soil as irrigation water, because of persisting below the permissible limit (20.00 mg L -1 ) for irrigation (Ayers and Westcot, 1985).
In the study area, trace amount of Fe and Mn (<0.01 mg L -1 ) were detected in all groundwater samples.Ayers and Westcot (1985), reported that the maximum recommended concentration of Fe and Mn in water used for irrigation is 5.0 mg L -1 and 0.20 mg L -1 , respectively that indicate the groundwater as fresh water and no ionic constituents are considered as hazardous to crop growth for irrigation

CONCLUSION
From the investigation, it can be concluded that groundwater of the study area would not create any problem for irrigation which related to soil properties and crop growth and that have not long term effects on irrigating agricultural crops, that could be safely used for irrigation purpose in Tangail district, hence all of the groundwater samples were within the standard limit.Finally it can be recommended that, the water quality must be tested with time interval for better planning of any irrigation system to sustain the crop production.

Figure 1 .
Figure 1.Map showing the study area at the four upazila in Tangail district(Banglapedia, 2008)

Table 1 .
The average value of physico-chemical parameters of groundwater for irrigation

Table 2 .
The average values of ionic constituents of groundwater for irrigation