N and P variation in Groundwater in Wet Zone and Dry Zone in Sri Lanka due to Fertilization of Paddy Crop

S.K. Gunatilake

Abstract


Nitrate and phosphates are the main contaminants in groundwater due to application of fertilizer in agricultural sector.
Therefore, present study was conducted to identify the general trend of fertilizer usage in the paddy sector and how much N and P
affect to underground water. A total of 142 groundwater samples were collected from drinking water wells, covering two different
climatic zones. The physico-chemical parameters were determined using standard procedures (APHA1992). pH and EC, fluoride,
nitrate, phosphates and sulfates were measured. The average NO
–N concentrations in groundwater before fertilization in wet and dry
zones are 1.47 and 0.98 mg/L. During the cultivation period farmers applied three times fertilizer their land and average value of NO
3
N
was
increased
up
to
7.8
mg/L
in

wet zone and 3.7 mg/L in dry zone groundwater. 17% of the groundwater samples were over the
maximum contaminated level of 10 mg/L as NO
-N before fertilization and it was increased up to 26% after fertilization in wet zone.
In dry zone nitrate concentration in all samples was less than the maximum limit before applying fertilizer. But during the cultivation
period 12% of the samples exceeded the WHO maximum contaminating level. The phosphate concentrations in all groundwater
samples were exceeded the WHO maximum contamination limits (0.01 mg/L) before and after fertilization. High EC, high TH and
high pH in the dry zone are notable compared to that of in the wet zone. However it was observed that timing and amounts for
fertilizer applied directly influence the potential groundwater contamination in different climatic zones in Sri Lanka. Therefore proper
management practices are needed where intensive agricultural activities are practiced.

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