LandDisposal of Wastewater

ReportNo WSAA 132

December 1997

SYNOPSIS

The specificstudy objectives were:

1.       To identify the flux ofnitrates/nitrites/nitrogen in the soil profile;

2.       To quantify the amount of nitrateleaching to the water-table

3.       To develop water and land managementoptions to remove or minimize the risk of groundwater contamination by nitrate.

Objective 1 and2 were fulfilled in part, with results identifying some nitrogen fluxes withinsoil and groundwater, and groundwater analyses identifying nitrogen leached tothe water-table. Objective 3 was not addressed due to insufficient informationon soil nutrient processes and water balance.

Key conclusionsthat can be drawn from this study are:

-         Gaseouslosses of nitrogen from the soil/water system are assumed to be verysignificant and may account for up to 50% of the applied nutrient load(assuming approximately 25% of the applied nutrient load is absorbed by thesoil or taken up by plants);

-         Deepdrainage to the groundwater represents more than 50% of the inflow volume yetaccounts for only 5% of the applied nutrient load.

-         Groundwaterlevels respond rapidly to wastewater application. Hydrographs from allmonitoring bores show a response of groundwater within minutes of wastewaterapplication. This response may be a result of bypass flow.

-         Rawwastewater irrigation resulted in measurable impacts on the groundwater at 2 mand 5 m depth. The variation in redox potential measured in groundwater andsoil indicate that microbial processes are occurring that respond to irrigationevents. The study, however, did not address the long-term impact of repeatedflood irrigation events.

-         The actionof surface runoff down the pasture bay during flood irrigation resulted in themobilization of soil nitrogen, mainly in the form of nitrite. This suggeststhat nitrite accumulation in the upper soil profile may result from floodirrigation practices.

This study didnot assess the soil storage nutrient dynamics which is critical in order toconfidently report nutrient balance closure. The partitioning between gaseouslosses and increase in soil organic nitrogen depends on estimates of long termincreases in soil organic nitrogen; the nutrient and phosphate balances arecomplicated by the absence of data about long term increases in soil nitrogenand phosphate.

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