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A few hints on health effects of  nitrate -

This page quickly provides only a bit of information about health effects of nitrate for those readers which are not yet informed about. Please read special literature for more details!
Nitrate itself is not considered as to be toxic. In the digestive tract, it can however be reduced to nitrite leading to the formation of methaemoglobine especially dangerous for (less than 1 year old)  babies. 5 "blue-baby syndromes" have been observed in an Ukrainian village of 3500 inhabitants in the year 2002. The figure shows that the problem is of high importance.

Together with secondary amines, nitrate forms nitrosamine, which is one of the most effective carcinogen compounds. This is why the nitrate concentration of drinking water in Ukraine is limited to 45 mg/L (ca. 10 mg/L N).

Some more hints including pesticides, published by various US offices, are put together here:
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Nutrient      (lifetime health advisory)  mg/L
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Nitrate (as N)              10.0
Nitrite (as N)                 1.0

* Lifetime health advisory is US EPA's specific level of chemical concentration in water that is acceptable for drinking over a person's life span.
 
 

Health Effects

Pesticide residues and nutrients in drinking water may cause health problems. Pesticides, if ingested in large quantities over a relatively short period of time may cause damage to the nervous system and internal organs of both animals and humans. Long term effects on humans are unclear and studies are ongoing.

Nutrient impairment of ground water by nitrate can endanger the lives of children under the age of one year. Infants can develop a potentially fatal condition known as methaemoglobinemia, or "blue baby syndrome." Within the infant's digestive tract, nitrate is readily converted to toxic nitrite in the oral cavity and the stomach. The nitrite is then absorbed through the infant's gastro-intestinal tract into the blood and there it bonds to ferric (Fe+3) iron found in blood methaemoglobin. The nitrite prevents the reduction of ferric iron back to the ferrous (Fe+2) form required in haemoglobin for the transport of oxygen by blood. With greatly decreased blood-oxygen carrying capacity, the infant's tissues become oxygen-starved, the blood turns dark-brown and the skin exhibits a bluish tint. Nitrate in ground water may also be toxic to adults. Any questions or concerns about health effects of specific pesticides can be directed to DHES or MDA.
"Concrete figures are given in the book of the geographer A. CAPCELEA. He writes that  in Moldawia  about 1 million people in rural areas are subject to the harmful influence of nitrates (over 300 mg per 24 hours).
Experimental investigation have shown that the continuous loading of the human body, especially of children, with increased doses of nitrates contribute to the onset of the tissue hypoxic state, to metabolism misbalance, immunity loss, etc. In affected localities, a big number of children show retarded biologic development (s. tab., source 3 from literature list below), with certain consequences in physical development and with excess body mass..

Physical development

Competition between nitrate and iodide with its admission into the thyroid.

Nitrate in the blood competes with the transport of iodide into the thyroid. Iodide is needed for the synthesis of the vital hormone thyroxin. With normal supply of iodide outweighing of this transportation mechanism adjusts  the competition of nitrate. However if the offer of iodide is scarce and the nitrate pollution high, the thyroid seeks to adjust the lack of iodide (which is intensified artificially by nitrate) by enlargement and new formation of thyroxin forming cells. If this is not successful,  iodine lack can cause damages.
 

Pesticide Residues and Nutrients

The Montana General Agricultural Chemical Ground Water Management Plan addresses pesticide and nutrient use and best management practices to implement to prevent impairment of ground water and surface water. The following contaminant levels are established concentrations of pesticides and residues that are allowed in water.

Maximum contaminant levels: A maximum contaminant level or MCL are the highest concentration of a contaminant allowable in a public water supply. These MCLs are specified in the National Primary Drinking Water Standards for the United States. There are no known adverse health effects that exist at the recommended MCLs. MCLs are based on a lifetime exposure with a consumption of 0.52 gallons of water per day. This provides good protection to all people using a public water supply.

Secondary maximum contaminant levels: A secondary maximum contaminant level has been established for contaminants that may affect taste, odor, colour and appearance of drinking water. These MCLs have also been established to prevent adverse health affects. Although secondary MCLs represent reasonable goals for drinking water, they are not federally enforceable. Yet, each state is encouraged to implement the secondary MCLs.

Health advisories: Health advisories are guidance documents issued by the Environmental Protection Agency. These health advisories contain information on health risks, water treatment technologies and specific levels of chemical concentrations in water that are acceptable for drinking. The EPA reviews available human data and experimental animal studies in evaluating potential human health effects. The health advisories are updated as new information becomes available.
 

Livestock Water

Livestock require a good water supply, both in quality and quantity. Generally speaking, the maximum level of contaminants allowable in livestock water is higher than those levels acceptable for human consumption. Safe levels of agricultural chemicals in livestock water, especially pesticides, has not been thoroughly examined. Nitrate poses the greatest health hazard to livestock (especially young animals).

Nitrate itself is not very poisonous, but it will become poisonous when it changes to nitrite in an animal's digestive system. Once converted it enters the bloodstream and reacts with hemoglobin to render it incapable of carrying oxygen in much the same manner as was previously described for infants. The animal will begin to show signs of a lack of oxygen such as labored breathing and a lack of coordination.

Nitrate poisoning is more likely to occur in ruminant animals such as cattle and sheep. Research also shows, horses are more susceptible than swine and poultry. When livestock consume both well water and feed which is high in nitrate, the health hazard becomes even higher.

Table 1: Effects of nitrate in water used by livestock (Jackson, 1983, Olson, N.D., United States Department of Agriculture, Soil Conservation Service, 1982).

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The programs of the Montana Extension Service are available to all people regardless of race, creed, colour, sex or national origin.

Issued in furtherance of cooperative extension work in agriculture and home economics; acts of May 8 and June 30, 1914. In cooperation with the U.S. Department of Agriculture, Andrea Pagenkopf, Associate Vice Provost for Outreach and Director Extension Service, Montana State University, Bozeman, Montana 59717.

Funding provided by the Montana Agricultural Chemical Ground Water Protection Act and Fertilizer Checkoff. Additional information may be found in the General Agricultural Chemical Ground Water Management Plan (GMP).

Literature:

1. Office of Drinking Water, Office of Water, U.S. Environmental Protection Agency (19990): Risk Assessment, Management and Communication of Drinking Water Contamination. - Seminar Publication EPA/625-4-89/024
2. CAPCELEA, A. (1996): The Republic of Moldova on the way to the sustainable development (edit. by SOROS Foundation, Moldova
3. National Strategic Program for Environmental Protection till 2010. - Chisinau, 1995

links for the evaluation of  pesticides see there!