3.1. Potential Effect of Land Burial of Livestock Carcass on Water Quality
Buried livestock undergo decomposition. During this process, nutrients, pathogens, and other components of the animal carcass are released into the environment. Because these substances are released into the surrounding environment, they may be broken down, transformed, lost to air, or otherwise immobilized so that they pose no environmental threat. Ritter and Chirnside concluded that the pollution from burial pits is similar to that from domestic septic tanks and can be controlled with legislation synonymous with on-site wastewater treatment regulation [
11]. For instance, the Dead Animal Disposal Act in Ontario, Canada, outlines three legal disposal methods for dead cattle, pig, sheep, goats, and horses: pickup by a provincially licensed collector; composting under 60 cm of organic substrate, such as sawdust or straw; and burying under 60 cm of soil and far away from all waterways [
4]. However, few studies have investigated the effect of carcass burial on groundwater quality. On-farm burial pits are typically constructed without liners, and any leachate produced may infiltrate into the soil and groundwater. To date, investigations of the groundwater quality impact due to animal carcass disposal have largely focused on poultry carcass disposal, and a limited number of routinely measured contaminants, including nutrients, chloride, and fecal pathogens, have been examined [
6]. Increased concentrations of ammonia, nitrate, chloride, and fecal pathogens in groundwater have been observed on farms with poultry carcass disposal pits [
11]. Yuan et al. examined the concentrations of conventional contaminants, including chemical oxygen demand, total organic carbon, total nitrogen, total phosphorus, and solids, as well as veterinary antibiotics and steroid hormones in leachate and electrical conductivity values over a period of 20 months after the land burial of cattle carcasses. They found that most contaminants were detected in leachate after 50 days of carcass decomposition, reaching a peak concentration at approximately 200 days and decreasing to baseline levels by 400 days [
12].
MacArthur et al. reported the average leachate levels of ammonium-N (3290 mg/L), alkalinity (9400 mg/L), biochemical oxygen demand (BOD; 12,700 mg/L), and chemical oxygen demand (COD; 20,400 mg/L) on a burial site with FMD mortalities [
13]. A field study investigated leachate quality after the burial of poultry, bovine, and pig carcasses in separate pits that were isolated from the surroundings with a 0.04-inch polyethylene liner [
14]. That study detected elevated levels of ammonium-N (12,600 mg/L), alkalinity (46,000 mg/L as bicarbonate), chloride (2600 mg/L), sulfate (3600 mg/L), potassium (2300 mg/L), sodium (1800 mg/L), and phosphorus (1500 mg/L) and relatively low levels of iron, calcium, and magnesium in leachate samples. These data provide crucial information on the potential for groundwater contamination resulting from the leachate produced by most on-farm mortality pits in the Unites States [
12]. During the 2001 FMD outbreak in the United Kingdom, approximately 61,000 tons of carcasses were disposed of at four mass burial sites, and groundwater vulnerability maps were used to locate suitable mass burial sites [
15]. Such an approach can identify potential on-farm burial sites to minimize the risk of environmental pollution and can offer viable and practical options to farmers for disposal of on-farm mortalities.
3.3. Effect of Pig Carcass Burial on Groundwater Quality
Taiwan is characterized by high temperatures and humid climate. The annual rainfall ranges from 1800 to 2400 mm and mainly occurs in June–September. The mean annual air temperature is 22.5 °C, and the mean monthly temperature ranges from 15 °C to 30 °C. In this paper, four pig carcass burial sites with various groundwater monitoring wells were studied, namely Linlo, Chawchou, Shinbei, and Pousan. The Pousan site is located in Hsinchu Prefecture in northern Taiwan, and the other three sites are located in Pingtung Prefecture in southern Taiwan. Before the FMD outbreak in 1997, the number of pigs reared for consumption in Pingtung was much higher than that in Hsinchu; thus, the number of pig carcasses was much higher in Pingtung than in Hsinchu (
Table 1). Pousan was the first area to present with the FMD outbreak in 1997, and its geographic condition is considerably different from that of Pingtung. Therefore, the Pousan site was also involved in the groundwater monitoring plan. The three monitoring sites of Pingtung are located in alluvial plains with fine-size Holocene sediments and exhibit high seasonal water tables ranging from more than 2.0 m to less than 0.5 m. In contrast, the Pousan site in Hsinchu is composed of coarse Quaternary gravel. All burial pits were 2–5 m in depth.
In addition to the potential introduction and subsequent survival of pathogenic bacteria in soil and water as a result of carcass burial, burial may lead to the proliferation of pathogens and the subsequent pollution of groundwater and drinking water. Many factors affect the transport of pathogens from soil to groundwater, including soil type, permeability, water table depth, and rainfall. However, adsorption, filtration, and predation by natural microbial populations significantly reduce the amount of pathogens transported to the underlying groundwater [
17]. The microbial and chemical characteristics of groundwater in the monitoring wells are shown in
Table 2. The average total coliform in Well K was much higher than that in Wells J and L at the Pousan site. However, no significant difference was observed in total coliform in wells at the other sites. The average fecal coliform at the Chawchou, Sinbei, and Pousan sites increased from upstream to downstream, indicating the negative effect of pig carcasses on groundwater quality. However, the FMD virus was not detectable in any samples throughout the period, because the virus could be inactivated by the heat and acidity of the organic acid-generating processes under the anaerobic conditions in the buried pits. In addition, it was difficult to evaluate the effect of the burial of pig carcasses on COD in groundwater because of the substantial variation in the data of the upstream and downstream wells. The nonpurgeable organic carbon (NPOC), total oil, and total dissolved solid (TDS) levels were clearly higher in the downstream wells than those in the upstream and control wells at all sites. The time sequence profiles of COD and TDS in Well K at the Pousan site indicated that these levels changed seasonally as follows: a slight increase in December 1997, rapid increase in January–March, gradual decline in April 1998, and rapid increase again in May 1998 (
Figure 5).
The chemical composition of groundwater in the monitoring wells at the four sites is shown in
Table 3. Elevated levels of BOD, NH
4-N, TDS, and Cl were commonly found within or very close to the burial pits of livestock carcasses. Although the chloride concentrations are generally lower than those of other contaminants, elevated chloride levels are the optimal indicator of burial-related groundwater contamination [
4,
18]. Moreover, chloride is a conservative ion that does not undergo significant oxidation/reduction reactions, adsorption on to mineral grains, or complexation [
19]. At the investigated sites, a peak concentration of chloride was observed in January and February 1998, but the chloride concentration declined after March 1998 (
Figure 6). The chloride concentration was significantly higher in the downstream wells, except for the wells at the Linlo site. The nitrogen concentration, particularly ammonium-nitrogen, was higher in the downstream wells than in the upstream wells (
Table 3). The sulfate concentration also increased from the upstream wells to the downstream wells at all sites. At the Pousan site, the average concentrations of Cu, HS, and Fe were not only higher in Well K than in Wells J and L, but also peaked in January and February 1998 (
Figure 6).
Several indicators of groundwater quality were selected and found to be associated with contaminants released from the pig burial sites. These indicators included total bacterial count, fecal coliform,
Salmonella spp., nitrite-N, nitrate-N, ammonium-N, sulfate, NPOC, total oil, and TDS. The soil particles were much finer at the Linlo, Chawchou, and Sinbei burial sites in Pingtung than at the Pousan site in Hsinchu. Thus, the groundwater quality shown in
Table 2 and
Table 3 indicates that the Pousan site was heavily contaminated compared with the other sites. In addition to soil texture, burial without an impermeable cloth and the relatively short distance of the burial site from the monitoring well may contribute to the contamination of groundwater at the Pousan site. The total bacterial count, fecal coliform, ammonium-N, iron, and copper (100 cfu·mL
−1, 100 cfu·mL
−1, 0.1 mg·L
−1, 0.3 mg·L
−1, and 1.0 mg·L
−1, respectively) in some of the monitoring wells exceeded the limits of drinking water standards of Taiwan. Therefore, the groundwater at these sites is not recommended for drinking. Appropriate disinfection and boiling of the water is required before drinking.