By Hydrosimulatics INC  

Pesticide Spill Problem

As a result of a labeling error, a banned water-soluble pesticide was accidentally applied one day over an agricultural area of around 1000 hectares. The application rate of the pesticide was about 0.5 kg per 1000 square meters.  The soils in this area are sandy and very permeable, resulting in very little surface runoff in the area. The underlying phreatic aquifer in this area is also sandy which receives recharge at a rate of 0.5 m/yr. The aquifer is drained by a small creek which sustains it low entirely as a result of subsurface drainage from the agricultural area (see Figure 1). Inhabiting the creek is the largest known population of a rare and endangered species of minnow which is particularly sensitive to environmental contamination. Concentrations of the pesticide greater than 50 ppb are known to be toxic to this species of minnow. 

Develop a quantitative analysis of the situation as follows:

  • If the water table is typically a distance bs=2.5m below the land surface, estimate the rate of movement down through the soil. Estimate the maximum concentration of the pesticide in the water as it reaches the water table, if dispersion can be assumed to be negligible. (Soil water characteristics for this soil are shown in Figure 2.)
  • Predict the time of variation of the concentration of the pesticide to be expected in the creek. The average aquifer thickness, baq, is estimated to be 50m.
  • Use the results of the above analyses and other pertinent information to develop recommendations on the solutions proposed to this problem as outlined below.

Figure 1: Creek and phreatic aquifer at risk to contamination from the pesticide application.

Proposed solutions:

  1. The state environmental agency proposed to do a 9 mo., $500,000 sampling and modeling study before recommending any action.
  2. Environmentalists propose that a soil-treatment process applied to the upper 20 cm of the soil be initiated immediately. This will cost several million dollars and involved tilling of the soil which would destroy the crops.
  3. The farmers have suggested that the problem be solved by capturing the minnows and holding in a fish hatchery for a few months until the creek contamination has cleared. 

For solution ii), determine how effective (% removal) would the treatment have to be to prevent harm to the minnows. Also determine how soon the treatment must take place.

For solution iii) determine how soon the minnows can be returned to the creek.

Figure 2: Soil moisture characteristic curves.