Nitrates are a common contaminant frequently found in drinking water. Nitrate itself is not carcinogenic, but instead acts as a "pro-carcinogen," meaning that it reacts with other chemicals in our bodies to form carcinogenic compounds. Nitrate contamination of drinking water can occur in areas where nitrogen fertilizers are used.
Naturally occurring nitrate levels in decomposing plants and vegetation can increase nitrate concentrations in the groundwater when it rains as well. Nitrates can be found in both areas of new construction (where houses are built on land previously used for farming) and can also be found in older wells where trees have grown around the well and have produced years of foliage drop.