## 3 Nitrate and Bacterial Contamination in Dairy Cattle Drinking Water  
Nitrate and Microbial Pollution Present in Dairy Cattle Drinking Water: Sources, Recommended Levels, and Health Impacts  
Manure produced on dairy farms is considered to be a large source of GHG emissions such as methane and nitrous oxide, but also, importantly, excessive amounts can be an acute source of certain pollutants such as nitrate and can contribute to elevated nutrient and bacterial loads in groundwater and surrounding surface water [2]. This can be detrimental to surrounding ecosystems as well as the people and animals relying on local water sources for drinking water. Dairy cattle predominantly drink water from troughs (as seen in Figure 1), and this water is often sourced from groundwater and is rarely treated before consumption. Two major pollutants in water that may pose risks to cattle are nitrate and _E. coli._ Elevated levels of nitrate in groundwater are often linked with the storage, handling, and disposal of livestock manure and agriculture more broadly (i.e., synthetic fertilizer inputs). Figure 1: Typical setup of dairy cattle drinking water trough. Algae present indicates nitrate present, and certain forms may pose toxic threats to cattle. Similarly, microbial pollutants such as elevated levels of _E. coli_ in water are caused by manure, and many outbreaks are linked with manure-borne pathogens. When manure is flushed or washed away as runoff, it can contribute to elevated levels of nitrate in surface water, and because of nitrate's high solubility, it can easily seep into groundwater [12]. The maximum contaminant level (MCL) of nitrate in human drinking water, set by the EPA, is 10 mg/L of NO3\({}^{-}\)N (as nitrogen) and 44 mg/L NO3\({}^{-}\) (as nitrate) because it can lead to harmful health effects such as methemoglobinemia, or blue-baby syndrome, which causes a baby's skin to turn blue as well as being linked to cancer and other health effects [13; 14]. Even though there are no regulatory standards for livestock drinking water, it is recommended that nitrate levels for cattle/livestock drinking water stay below 100 ppm. The U.S. EPA's standard for a safe level of nitrate (\(\approx\)10 ppm) in drinking water for humans is considered to be safe for livestock also [15]. This is largely because--similar to humans--nitrate can reduce to nitrite in the cow's system, which decreases the blood's ability to transport oxygen [16]. Acute nitrate concentrations have dangerous consequences for cattle health (in some cases even leading to death), but long-term chronic toxicity is a more common issue [5; 6]. Calves and lactating cows are especially susceptible to high levels of nitrate. It has been shown to decrease their water consumption and, in turn, negatively impact their milk supply in addition to negatively affecting fertility [8]. The risk from nitrate ingestion is additive, so limiting nitrate in drinking water is especially important when high levels of nitrate are present in feed. High levels in feed may be due to several factors such as the type of crop species having higher nitrate accumulation potential (i.e., sorghum), high rates of nitrate application in the form of fertilizers, and conditions of limited plant growth such as droughts [15; 16]. Both nitrate and phosphate--which is also found in manure--can lead to eutrophication in surface waters, which is the excessive growth of algae and the subsequent decrease in dissolved oxygen, which is harmful for aquatic ecosystems [17]. High levels of nitrate in groundwater are not only an issue when directly consumed, but nitrate and phosphate can also contribute to algae growth in trough waters, especially in sunny and warm conditions (Figure 1). The presence of algae can negatively affect the odor and taste of the water, which is known to decrease the amount of water consumed by cattle and can decrease the amount of milk produced [18].