Nitrates in Dug Well Water and Their Removal

Nitrates in water are chemical substances that can enter water from fertilizers, wastewater, or industrial sources. High levels of nitrates in drinking water can be dangerous to health, especially for infants and pregnant women, as they can impair the body's oxygen transport. Water tests help determine their level.

The most significant problem with groundwater is excessive concentrations of iron compounds, sometimes manganese, ammonium, or hydrogen sulfide, whereas this problem does not exist in well and spring water; here the main and biggest problem is nitrates (microbiological contamination of well water will not be discussed here). Nitrates have another name (synonym) widely used by farmers and rural people – saltpeter. Potassium nitrate is called potassium saltpeter, ammonium nitrate – ammonium saltpeter, and so on.

What are nitrates, why is there so much said and written about them, and are they really as terrible and dangerous as all that? Nitrates are one of many forms of nitrogen compounds, and nitrogen, together with phosphorus and potassium, is one of the most important biogenic elements. Biogenic elements are simply essential for the normal existence of living organisms. Without saltpeter, as well as potassium and phosphorus fertilizers, it is impossible to grow a good harvest. However, nitrates (saltpeter) have another very important property – nitrates of all metals are highly soluble in water, and the portion that living organisms (microorganisms and plants) fail to take up in time for protein synthesis migrates easily and enters groundwater. This is how nitrates come to be in the water of wells, springs, and shallow boreholes.

Hygienists have found that, once inside the human body and acted upon by various enzymes, nitrates first turn into much more dangerous nitrites, and these in turn into nitroso compounds. Tests on various animals have shown that nitroso compounds are already carcinogenic and promote the formation of malignant tumors. Nitrites act in a similar way to carbon monoxide fumes (synonym – carbon monoxide, CO), i.e. they prevent hemoglobin from supplying the body with oxygen.

Taking into account the recommendations of WHO (World Health Organization) specialists, every country in the world strictly limits nitrite and nitrate concentrations in its drinking water standards. Under Lithuanian drinking water hygiene standard HN 24: 2003, the nitrite concentration in water must not exceed 0.5 mg/L, and nitrates – 50 mg/L (10 mg/L for infants).

Those residents of Lithuania who get their drinking water from centralized water supply systems practically never encounter the nitrite and nitrate problem. However, a significant proportion of Lithuania's rural residents still use dug well water for drinking and cooking. According to the Ministry of Environment, there are currently about 300 thousand dug wells in Lithuania (shallow wells 2–5 m deep), and about a third of Lithuania's population drinks water from them. According to the same ministry, roughly half of these wells have significantly elevated nitrate concentrations. The author of this book has had occasion to test the water from well over a hundred wells, brought in from various districts of Lithuania. The results are disheartening. Only in a small proportion of wells was the nitrate concentration in the water below 50 mg/L. Very often nitrate concentrations exceeded the permitted standard by 2–3 times or more. There was even a record-holding well, where the nitrate concentration reached 850 mg/L. Such water is fine for watering a vegetable garden — the harvest is guaranteed — but drinking it is genuinely dangerous. It is not uncommon for well owners to have no idea what quality of water is in their wells, or what the nitrate concentration there actually is.

How do nitrates get into well or shallow-borehole water? The first and constant source of nitrates is the atmosphere. Burning any fuel in furnaces, boiler houses, car engines, etc. requires the oxygen present in the air. Along with oxygen, nitrogen also enters the combustion chambers from the air, and there is considerably more nitrogen in the air than oxygen. At high temperatures, part of the fairly inert (inactive) nitrogen is converted into nitrogen oxides, which enter the atmosphere together with other combustion products. The nitrogen oxides then continue to transform in the atmosphere, and the resulting nitrates reach the earth's surface with precipitation.

The second source is mineral and organic nitrogen-containing fertilizers. Fertilizing is usually done in spring or in early summer.

The third source is the decomposition of nitrogen-containing organic matter in soil and water.

It has been established that nitrate concentrations in well water change with the seasons. The highest nitrate concentrations are usually recorded in spring and early summer, and the lowest in late autumn and winter.

In 1992–1994, i.e. for three years, the author observed the seasonal fluctuations of nitrate concentrations in two dug wells in a gardeners' community on the bank of the River Vokė. Chemical analysis was carried out at least twice a month (sometimes more often). The average results for each month are given in Table 1 and Figure 1.

Table 1. Seasonal changes in nitrate concentrations in well water

Month	Average nitrate (NO3­– ion) concentration, mg/L
Well 1	Well 2
January	32	17
February	34	19
March	45	28
April	68	49
May	95	63
June	83	55
July	57	42
August	51	40
September	45	31
October	42	27
November	38	22
December	34	21

Fig. 1. Seasonal fluctuations in nitrate concentrations in well water

We can see that nitrate concentrations in the water of both wells changed in a similar way. The highest concentrations were recorded in April, May, and June (with a maximum in May), and the lowest in November, December, January, and February. The results of observations from the other two years were very similar to those shown in the table and figure, and are therefore not presented here.

Incidentally, seasonal fluctuations in nitrate concentrations were also observed in the water of springs flowing at various locations. The pattern of seasonal change in concentrations is very similar to that in wells, except that nitrate concentrations in spring water are usually considerably lower than in well water and generally do not exceed the requirements of drinking water hygiene standard HN 24:2003.

Why are the highest nitrate concentrations in the water found in April, May, and June, and the lowest in November, December, January, and February? In the author's opinion, a sufficiently logical explanation would be as follows. In winter, once the ground has frozen, nitrates do not migrate into groundwater. Fields are not fertilized in winter, and nitrates reaching the ground from the atmosphere accumulate on its surface. In spring, once the frost has gone, the nitrates that built up on the surface over winter travel down into the groundwater and, with it, into the wells. At the same time, intensive agricultural work begins, along with intensive fertilizing of fields with organic and mineral fertilizers. This is when nitrate concentrations in well water reach their maximum values. As the weather warms further still, an intensive vegetation period begins for plants (and various microorganisms), during which nitrate nitrogen is used for protein synthesis, and nitrate concentrations in the soil and groundwater fall rapidly.

Therefore, in the author's opinion, checking, how much nitrate is in well water should be done in spring and early summer. If nitrate concentrations in the water do not exceed the hygiene standard requirements (≤50 mg/L) at that time, then it can practically be guaranteed that there will be even less at other times of the year.

Now we should discuss how to remove nitrates from well water, or at least reduce their concentrations. If you talk to different people on this subject, you can hear all sorts of opinions, suggestions, recommendations, and recipes, starting with the idea that wells should be covered with lids; that a protective clay layer 1–2 meters in diameter and at least 1 meter thick should be built up around the well; or that chlorinated lime should be poured into the well and the water then bailed out; or that the nitrates should simply be bailed out of the well. Unfortunately, none of the methods mentioned here reduces nitrate concentrations in well water. After all, a well is not a tank in which nitrates collect and accumulate. Nitrate concentrations in well water are the same as in the surrounding groundwater that feeds the well. If it were possible to reduce nitrate concentrations in the groundwater feeding a well, nitrate concentrations in the well water itself would automatically drop to the same level. How can this be done? There would need to be a sanitary zone of a certain size around the well (it is hard to say exactly what size, but in the author's opinion, a considerable one), in which there is no intensive farming, no barns with manure piles kept beside them, and so on. In other words, the well should be built well away from farm buildings and intensively worked land. However, this is not convenient, and so wells are usually built as close to the house as possible. On the other hand, even after building a well away from your own land, it is still not clear, whether nitrates will decrease. After all, it is not clear, which paths groundwater travels by. It is quite possible that groundwater, fed with nitrates from your neighbors, is exactly what ends up in your well.

A guaranteed way to get rid of water containing a lot of nitrates is to install a deeper borehole. In this case, however, other problems will arise: the borehole water will most likely contain iron, and it may have elevated concentrations of manganese and ammonium. In this case, water treatment equipment will again need to be installed.

Another guaranteed way to remove nitrates is filtering water at a suitable rate through an anion-exchange filtering medium, for example a weakly basic anion exchanger in chloride form (R–R3NCl). These anion exchangers remove NO3–, SO42–, and many other anions well:

R–NH3Cl + NO3– → R–NH3NO3 + Cl–,

2 R–NH3Cl + SO42– → (R–NH3)2SO4 + 2Cl–.

We can see that the Cl– ion concentration in the filtrate increases equivalently, i. e. after removing 1 meq. of NO3– or SO42– ions from the water (62 mg of NO3– or 48 mg of SO42–), the Cl– concentration in the filtrate increases by 1 meq. (35.5 mg). It is easy to calculate that after removing 250 mg (4.03 meq.) of NO3– ions from the water, 4.03 meq. (143 mg) of Cl– ions pass into the filtrate. Once the anion-exchange capacity is exhausted, the weakly basic R–NH3Cl anion exchangers are regenerated with a solution of table salt, i. e. NaCl, solution:

R–NH3NO3 + NaCl → R–NH3Cl + NaNO3  (molecular equation)

R–NH3NO3 + Cl– → R–NH3Cl + NO3–  (ionic equation).

The concentration of nitrites in natural water is usually very low because of their instability (they are just as easily reduced to ammonium ions as they are oxidized to nitrate ions). Nitrites are hard to detect in clean water, since they occur in thousandths of a milligram per liter. Somewhat more is found at the end of the vegetation period, when organic matter is present. Nitrites are an intermediate part of the nitrification process.

Studies on nitrate removal using a weakly basic anion exchanger were carried out at the Analytical Chemistry Laboratory of the VGTU Department of Chemistry and Bioengineering. A glass column 4 cm in diameter was loaded with 100 g of Japanese-made weakly basic chloride-form anion exchanger R–NH3Cl. Water from the Vilnius Antaviliai water supply source, which met hygiene standard requirements, was used for filtering; using NaNO3, an NO3– ion concentration of 107 mg/L was created in it. Filtration rate ~5 m/h. Filtering continued until the anion-exchange capacity was fully exhausted and the NO3– ion concentration in the filtrate returned to the initial level of 107 mg/L. The results of this study are shown in Fig. 2. The study showed, that under these conditions nitrates can be removed from 40 liters of water. When more water is filtered, the nitrate concentration in the filtrate begins to rise and returns to the initial level after 60 liters of water have been filtered.

After regenerating the anion exchanger, the study was repeated. The results obtained in the second study were almost identical and are therefore not presented here.

Fig. 2. Nitrate removal using chloride-form (R-NH3Cl) anion exchanger

After the second study was completed, the anion exchanger was regenerated not with NaCl, but with a solution of NaHCO3 (baking soda), i. e. instead of the chloride-form anion exchanger R–NH3Cl, a hydrogen-carbonate-form anion exchanger R–NH3HCO3 was obtained. When water is filtered through the R–NH3HCO3-form anion exchanger, in the exchange reaction, in place of Cl– ions it is HCO3– ions that take part, and it is these that pass into the filtrate. Thus, in the filtered water it is not the Cl– concentration that increases, but the HCO3– ion concentration. And this is a good thing, because HCO3– ions give the water a good taste, and their concentration is not regulated by the hygiene standard. The results of this study are shown in Fig. 3.

Fig. 3. Nitrate removal using hydrogen-carbonate-form (R-NH3HCO3) anion exchanger

Comparing the data given in Fig. 2 and Fig. 3, it can be seen that removing nitrates with the two different forms of anion exchanger produced almost identical results.