THE USE OF EPIDEMIOLOGIC CONCEPTS AND TECHNIQUES TO DISCERN FACTORS ASSOCIATED WITH THE NITRATE CONCENTRATION OF WELL WATER ON SWINE FARMS IN THE USA

This epidemiological study investigates the relationship between vario us factors associated with swine farms and the nitrate concentration o f well water in the USA. Through a random sampling procedure, 605 swin e farms located in 18 states were selected for inclusion in this study . A total of 631 well water samples were collected from these farms an d tested for a variety of elements and compounds. The concentrations o f nitrate, nitrite, sulfate, chloride, sodium, potassium, ammonia, flu oride, bromide and lithium were determined by an ion chromatograph whi le an inductively coupled argon plasma emission spectrophotometer was used to determine the concentrations of calcium, magnesium, barium, zi nc, iron and phosphate. Data concerning various farm factors were gath ered via a personally administered questionnaire. The data were examin ed using both multiple linear regression and logistic regression. Resu lts indicate that 53.6% (338/631) of the wells contained detectable le vels of nitrate, 11.7% (74/631) had nitrate levels exceeding 45 ppm an d 4.3% (27/631) exceeded 100 ppm. Logistic models demonstrated an asso ciation between nitrate concentrations > 45 ppm, increasing water pota ssium levels and wells < 100 ft deep. Nitrate levels > 100 ppm were re lated to increasing water concentrations of potassium, magnesium, bari um and zinc, wells 6-10 years old, increasing distance from the study farm to the nearest cattle farm and a greater distance to the nearest waterway located off the study farm. A negative association was seen b etween nitrate concentrations > 100 ppm, the water level of sulfate, a nd the use of the same well to supply both the household and livestock . Multiple linear regression models revealed a positive association be tween increasing nitrate concentration and the water levels of chlorid e, calcium, zinc and the greater number of miles from the study farm t o the nearest farm with cattle or sheep. A negative association was no ted between the concentration of well water nitrate and the water leve ls of sulfate and ammonia, the use of water treatment, the number of m iles to the nearest farm with poultry, the employment of water treatme nt and the use of the same well to supply water to both livestock and the household.