Assessing the Water Pollution Risk by Nutrient Salts Efflux from Farmland

Background and Objectives

While application of fertilizers and organic materials on farmland is indispensable for stable food production, it is also the cause of water pollution by nitrates, phosphates and other nutrient salts deriving from the applied substances. The number of water wells that do not conform to the environmental standard for nitrates concentration is on a constant rise. The revised Lake Law requires that effective measures to reduce nitrates and phosphates efflux from farmland should be selected and introduced, subject further to subsequent verification for effectiveness. It is very necessary, therefore, to develop methodologies that will permit an accurate assessment of water pollution risks by nutrient salts efflux and the optimal selection of effective efflux reduction technology. NIAES is conducting research to clarify the behavior of nitrates and phosphates from farmland soil through groundwater and river basin, with the objective of developing assessment methods of water pollution risks by nutrient salts.

Project Outline

In our research, we look at nitrate nitrogen and phosphor as our major research targets and aim to clarify how these nutrient salts move through soil, river and other surface water and groundwater, and to develop models for water pollution risks by nutrient salts at the level of river basins.

Not the entire amount of nitrogen fertilizer applied to a farmland, less what has been absorbed by the crop, escapes as nitrate nitrogen (NO₃^-) into groundwater and rivers to cause water pollution. It is known that, in a reductive environment where oxygen (O₂) is in short supply, nitrate nitrogen is consumed in connection with microbial decomposition of organic matters (a phenomenon called “denitrification”) (Fig. 1). In an effort to understand the soil and topographical conditions under which this denitrification occurs, as well as the distribution of locations in the farmland soil where the denitrification process takes place, we are developing methods to quantitatively assess the denitrification function in groundwater, by means of groundwater quality monitoring and the change in nitrogen isotopic composition.

In contrast to nitrate nitrogen, phosphor by nature is susceptible to adsorption by soil particles and its concentration in soil solution is generally low. For these reasons, it has been considered that phosphate efflux from farmland is predominantly due to soil loss by rain whereby phosphates flow out to rivers, adsorbed by soil particles. More recently, however, it has been made clear that phosphates adsorbed by soil particles do flow out into underground ducts (drain ducts buried below the upland field surface) through the cracks formed by the shrinkage of drying clay soil in a field that has been converted from a rice paddy to an upland field. Accordingly, we are investigating such outflow of phosphates from farmland and are developing methods to assess the water pollution risks of phosphates.

In order to determine effective measures to reduce water pollution by nitrogen and phosphor from farmlands and to predict their performance, risk assessment of water pollution by these nutrient salts at the river basis level is needed. We are therefore developing models to predict the dynamics of these nutrient salts in the river basin. We are also carrying out research to clarify the soil, topographic and weather conditions under which water pollution by nitrogen and phosphor are likely to take place, with the objective of drawing water pollution risk assessment maps.

Relevant Outcomes to Date

We have conducted a close survey on groundwater quality and soil denitrification activity in a toposequence comprising upland tea plantation and lowland rice paddies. The survey revealed that the soil strata in which elimination of nitrate nitrogen progresses most actively by denitrification during the groundwater streaming process are locally distributed in the vicinity of the border between diluvial and alluvial soils, and consequently, nitrate nitrogen concentration in groundwater is substantially lowered.

We have also clarified that, in the clay soil of a converted upland field in which cracks have developed in the lower soil layer, rainfall causes the efflux of phosphates adsorbed by soil particles into underground ducts, and that this efflux accounts for the major portion of the phosphate efflux into drainage (Fig. 2).

In addition, we are developing a nitrogen load computation model by the numerical conversion of land use situations and the incorporation of a Geographical Information System (GIS), for the purpose of quantitatively assess the nitrogen elimination function in accordance with the mode of land use in the basin.