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Assessment of Non-point Source Pollution from Croplands into the Manila Bay in Compliance to Supreme Court Final Order for the Manila Bay Cleanup, Rehabilitation and Restoration

By Dr. Edna Samar – June 05, 2012



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Map 1. Map showing the four (4) groupings of sub-watershed

In compliance to the Supreme Court Final Order dated February 15, 2011 stipulating the Department of Agriculture as one of the mandamus agencies for the Manila Bay Clean Up, Rehabilitation and Restoration, the BSWM spearheaded the generation of baseline data for the estimation of the pollution loading from croplands. The Soil and Water Resources Research Division (SWRRD), Water Resources Management Division (WRMD), Agricultural Land Management and Evaluation Division (ALMED), Cartographic Operations Division (COD) and Integrated Soil Resources Information Services (ISRIS) joined together to complete the study.

The watershed approach was used in the assessment and generation of baseline data on nutrient/ pollution loading from croplands into the Manila Bay system. Sub-watersheds were identified and grouped into four (4), namely: Pampanga river basin, Bataan watershed, Pasig river basin (Pasig-Marikina and Pasig-Laguna), and Cavite watershed. Characterization was done in terms of land use and vegetation, soil and water quality including microbiology. Survey and mapping were done using the dominant-associated scheme. The land use and vegetation map was overlaid with the watershed map to determine the extent and distribution of specific uses within the sub-watershed.

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Map 2. Land Use Map of the Manila Bay

Sampling points were identified along the river tributaries with considerable size of agricultural areas. Water samples were collected and brought to different laboratories for nutrients (BSWM), heavy metals (PCA), pesticide residue (BPI-NPAL) and bacteriological analyses (BSWM). Soil samples were collected and subjected to physico-chemical analyses including heavy metals, pesticide residue and bacteriological analyses.

The Food and Agriculture Organization of the United Nations (FAO-UN) soil taxonomic classification was adopted in soil mapping. Available historical data on chemical fertilizers and pesticides utilization from other agencies (Bureau of Agricultural Statistics, Sugar Regulatory Administration and Philippine Coconut Authority) were used to determine for the estimation of nutrient loading. Available literatures were reviewed and evaluated for the needed coefficients in the estimation of nutrient loading and were compared with standards/ acceptable limits to assess contribution of croplands into the pollution of Manila Bay.

The estimation of nutrient loading covered the croplands only within the sub-watersheds particularly rice, corn, sugarcane and coconut areas. These four crops covered 88% of the aggregate croplands of the Manila Bay system.

Results

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Figure 1. NH4 – N Nutrient Loading Contribution (kg/day) of the Four Manila Bay Sub-Watersheds
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Figure 2. NO3-N Nutrient Loading Contribution (kg/day) of the Four Manila Bay Sub-Watersheds
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Figure 3. Total P Nutrient Loading Contribution (kg/day) of the Four Manila Bay Sub-Watersheds

The entire watershed consists of four (4) groups of sub-watersheds: Pampanga River Basin, Bataan Watershed, Pasig River Basin and Cavite Watershed (Map 1). The importance of the Pampanga River Basin draining into the Manila Bay is noted as it comprised 63% of the entire watershed area of 1,972,014 hectares. The agricultural areas occupied 868,129 hectares, representing 45% (including fishponds) of the aggregate watersheds:

Distribution are as follows:

Croplands - 813,943 ha
Livestock and poultry - 3,808 ha
Fishponds - 50,378 ha

The nutrient loading from rice, corn, sugarcane and coconut became the focus of the analysis for this report. The pH, EC and TSS values from the different water sampling points are within the prescribed limits set by DENR except for the Naic, Cavite site where EC level was noted to be high considering its brackish condition.

The pollution load of, NH4-N, N03-N and Total P were estimated using the formula:

Pollutant Load = River discharge (cm) X concentration of nutrient (g/m3) X conversion factor (86.4)

The NO3-N, NH4-N and Total P were within limits/ criteria. Of the total observed NH4-N loading of 1,245.24 kg/day contributions are as follows: Pampanga River Basin, 482.89 kg/day; Pasig River Basin (372.82 kg/day); Bataan Watershed (274.58 kg/day); and Cavite Watershed (114.95 kg/day). From an aggregate NO3–N loading of 4,526.03 kg/day, Bataan sites topped in the NO3–N loading with 1,788.88 kg/day, closely followed by Pampanga River Basin. In the case of Total P loading of 1,876.69 kg/day, Pasig River Basin contributed the most at 861.24 kg/day. Pesticides residues in water samples particularly pose no harm as their observed values were below the limit of quantification (LOQ) set at 0.1 ul/L.

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Map 3. Major crop uses within the Manila Bay system

The pesticide residue analyses of soil samples showed no detriments as current levels of organochlorine, organophosphates and pyrethoids are less than the limit of quantification (LOQ) at 0.005 mg/kg. Lastly, all the samples collected from the four (4) sub-watersheds gave low levels of fecal coliforms in comparison with the standard criterion set by DENR for Class SB water.

In terms of the total coliforms, many sites exceeded the DENR standard for Class SB water. Notably, a very high concentration of total coliforms were obtained in Tanza, Cavite with the level reaching as high as 11,000 MPN/100 ml of sample. This was followed by Morong, Rizal with 4,600 MPN/100 ml.

The assessment of the N loss to the environment covered 88 percent of the croplands and these include ricelands, corn lands, sugarcane farms and coconut areas. In summary, the 712,510 hectares of croplands devoted to these crops utilized a total N of 52,102 mt. of which, 26,491 mt or 51% was loaded to the environment. Of this amount, 482 to 6,264 mt of N was lost as surface runoff from rice cultivation. The N use is still below the recommended fertilizer rates for rice and corn. In the case of coconut, salts are applied in lieu of chemical fertilizers, so that N use represents trace value. Improving the nitrogen efficiency would help lessen the N load to the environment while helping farmers to reduce wastage inputs.

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From the two sets of water samples, the croplands of the Manila Bay has minimal contribution to the nutrient loads of NO3-N, NH4-N and Total P to the water body as the existing load is way below the threshold limit on pollution loading. From the anthropogenic view, current uses of commercial fertilizers (in terms of N and P) and pesticides within the sub-watershed are below the recommended levels and thus, support the results of nutrient loading from water and soil analyses.

Summary and Recommendation

This study provides the baseline data on nutrient/ pollution loading from croplands into the Manila Bay system. Using the watershed approach, 18 sub-watersheds were identified, namely: the Pampanga River Basin, Bataan Watershed, Pasig River Basin and Cavite Watershed. The former is the largest occupying 63% of the entire watershed. The croplands comprised 46% of the entire watershed of the Manila Bay. The croplands are extensive in the Pampanga River Basin. Rice, corn, sugarcane and coconut covered 712,510 hectares, equivalent to 88% of the croplands.

In terms of nutrient loading from use of chemical fertilizers in the croplands, the aggregate N use from these areas is estimated at 52,102 metric tons where 26,491 mt or 51% of N was loaded to the environment. Of this amount, 482 to 6,264 mt of N was lost as surface runoff from rice cultivation. These aggregate N lost to the environment is based on levels yet below the recommended fertilizer rates for rice and corn.

In terms of pesticides residue, all the water and soil samples (representing the rice, corn, sugarcane and coconut areas) revealed levels below the limit of quantification indicating no danger to life. Based on the results, it is recommended that immediate efforts on improving the nitrogen efficiency of crops will lessen the N load to the environment and at the same time help farmers to reduce wastage of chemical inputs.





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