Environmental Impact of livestock mortalities burial

Abstract

The objective of this thesis was to determine the potential impact on groundwater quality as a result of the release of leachate from livestock mortality burial for three species of livestock: swine, bovine and poultry. Specific objectives were to:

1.Characterize the chemical composition of leachate in livestock mortality burial pits for three species: bovine, swine and poultry; and

2.Evaluate the potential environmental impact of livestock burial through groundwater transport modelling.

A two part program was followed to achieve these objectives. The first portion involved construction of lined burial pits complete with leachate collection systems. Poultry (1300kg), swine (5900kg) and bovine (9750kg) carcasses were each placed in separate pits and the pits covered with plastic liner material and then approximately one meter of earthen cover. The pits were sampled for leachate chemical analysis at 2 weeks, 1 month, 2 months, 4 months, 8 months, 14 months and 25 months post burial. The second portion involved using the chemical analysis results from first portion and two groundwater modeling software packages (CTRAN and PHREEQC) to characterize the leachate and evaluate the potential this material could have on groundwater resources adjacent to burial pits.

The results indicated that livestock mortality leachate contains, on average, after two years of decomposition, concentrations of 12,600 mg/L of ammonium-N, 34,600 mg/L alkalinity (as bicarbonate), 2,600 mg/L chloride, 3,600 mg/L sulphate, 2,300 mg/L potassium, 1,800 mg/L sodium, 1,500 mg/L phosphorus along with relative lesser amounts of iron, calcium and magnesium. Maximum values for the major ions were up to 50% higher than the average in some instances. The pH of the leachate was near neutral. In comparison to earthen manure storages and landfills, the strength of the leachate was 2-4 times higher.

To properly characterize the leachate chemistry, speciation of the mortality leachate was performed using PHREEQC. This speciation provided evidence of phosphate compounds precipitating from solution, as well as significant amounts of phosphoric acids (0.03 mol/L). Relatively high concentrations of ammonium sulphate also formed and due to the negative charge, allow for potentially 300 mg-N/L to transport conservatively. In comparison to naturally occurring groundwater, activities of bicarbonate, sulphates, phosphates and other minerals were many orders of magnitude higher than concentrations present in groundwater.

Preliminary simulations were created with two software packages, Geo-Slope CTRAN and PHREEQC to simulate transport of the leachate for three different soil conditions. The Geo-Slope model models a conservative contaminant, while the PHREEQC model involves geochemical speciation and contaminant transport including ion exchange occurring along the pathway. Transport through a low permeable soil (K=1 x 10-10 m/s) was dominated by diffusion allowing unattenuated leachate to transport a distance of approximately three meters in 50 years. The moderately permeable soil situation (K=1 x 10-9 m/s) produced a transport depth of six meters with an approximate concentration of the tracer thirty to forty percent of initial concentration in 50 years. In a highly permeable soil (K=1 x 10-8 m/s), transport reached a depth of 10 meters in 10 years with approximately forty percent of initial concentration. The PHREEQC transport model demonstrated a highly concentrated calcium and magnesium plume forming in front of the ammonium plume suggesting ion exchange and attenuation of ammonium.

In the occurrence of a mass mortality event, regulators in Canada have decided to employ a trench burial system. Trenches could be created using on-the-farm equipment such as backhoes to obtain approximate trench dimensions of 2 m wide and 4 m deep. To assess the impact of multiple trenches and their appropriate spacing, models were created with Geo-Slope CTRAN to evaluate the effects on trench spacing. It was determined through these models that a minimum 10 m separation distance would provide a potential contaminant plume maximum soil contact and no trench-to-trench impact.

To further evaluate the potential impact of livestock burial leachate, mass loading into an aquifer was evaluated for a moderately permeable soil (K=1 x 10-9 m/s) for a mass mortality event in a 10,000 head feedlot. Disposal consisted of ten 200 m trenches with a 10 m separation distance. Disposal covered 2.2 hectares and provided a mass loading of ammonium to an aquifer 10 m below of 950 kg/year after 50 years and increasing from 50 years until the peak concentration of the plume reached the aquifer. At this loading rate, nitrogen concentrations exceed drinking water standards 10-15 times.