Nitrates in well water and their removal

Nitrates in water – Chemical substances that can enter water from fertilisers, sewage, or industrial sources. High levels of nitrates in drinking water can be dangerous to health, particularly for infants and pregnant women, as it can disrupt oxygen transport in the body. Water testing helps to determine their levels.

The main problem with groundwater is excessive concentrations of iron compounds, sometimes manganese, ammonia or hydrogen sulphide, whereas in well and spring water this does not exist; here the main and biggest problem is nitrate (We will not be discussing microbiological contamination of well water here). Nitrates have another name (synonym) that is widely used by farmers or rural people – Saltpetre. Potassium nitrate is called saltpetre, ammonium nitrate is called ammonium saltpetre, and so on.

What are nitrates, why are they talked and written about so much, and are they really that terrible and dangerous? Nitrates are one of many forms of nitrogen compounds, and nitrogen, along with phosphorus and potassium, are the most important biogenic elements. Biogenic elements are simply essential for the normal existence of living organisms. Without saltpetre, as well as potassium and phosphorus fertilisers, it is impossible to grow a good harvest. However, nitrates (saltpetre) have another very important property – nitrates of all metals are readily soluble in water, and the part that living organisms (micro-organisms and plants) are unable to absorb for protein synthesis easily migrates and enters groundwater. This is how nitrates end up in the water of wells, springs, and shallow boreholes.

Medical hygienists have determined that upon entering the human body and being exposed to various enzymes, nitrates are first converted into much more dangerous nitrites, which in turn form nitroso compounds. Experiments carried out on various animals have established that nitroso compounds are indeed carcinogens and promote the occurrence of malignant tumours. Nitrites act similarly to carbon monoxide (CO), meaning they impede haemoglobin's ability to supply the body with oxygen.

Taking into account the recommendations of WHO (World Health Organisation) specialists, all countries in the world strictly limit the concentrations of nitrites and nitrates in drinking water standards. According to Lithuanian Drinking Water Hygiene Standard HN 24:2003, the concentration of nitrites in water must not exceed 0.5 mg/L, and the concentration of nitrates – 50 mg/L (10 mg/L for infants).

Residents of Lithuania who receive drinking water from centralised water supply systems practically do not encounter the problem of nitrites and nitrates. However, a significant proportion of Lithuanian village residents still use water from shaft wells for drinking and food preparation. According to the Ministry of Environment, there are currently about 300,000 dug wells (shallow shaft wells 2-5 metres deep) in Lithuania, from which about a third of the Lithuanian population drinks water. According to the same ministry, nitrate concentrations are significantly increased in approximately half of the wells. The author of this book has had to test several hundred well waters brought from various regions of Lithuania. The results of the tests are disappointing. Only a small proportion of well waters had nitrate concentrations below 50 mg/L. Very often, nitrate concentrations exceeded the permissible limit by 2-3 times or more. There was also a record-breaking well, where the nitrate concentration reached 850 mg/L. Such water is suitable for watering vegetable gardens and guarantees a harvest, but drinking such water is actually dangerous. It is not uncommon for well owners to be unaware of the quality of the water in their wells and the nitrate concentrations present.

How do nitrates get into well or shallow borehole water? The primary and constant source of nitrates is the atmosphere. The combustion of any fuel in furnaces, boilers, car engines, etc., requires oxygen from the air. Along with oxygen from the air, nitrogen, which is present in the air in much greater quantities than oxygen, also enters combustion chambers. At high temperatures, a portion of the relatively inert nitrogen is converted into nitrogen oxides, which, along with other combustion products, are released into the atmosphere. Subsequently, the nitrogen oxides further transform in the atmosphere and, having formed nitrates, reach the Earth's surface with precipitation.

The second source is mineral and organic nitrogen fertilisers. Fertilisation is usually carried out in spring or 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, while the lowest are in late autumn and winter.

During 1992–1994, i.e. for three years, the author monitored the seasonal variations in nitrate concentrations in two shaft wells in a horticultural society on the bank of the Vokė River. Chemical analysis was performed at least twice (sometimes more frequently) per month. The average results for each month are presented in Fig. 1. in the table and Fig. 1.

Table 1. Seasonal variations in nitrate concentrations in well water

Figure 1. Seasonal variations in nitrate concentrations in well water

We observed that the nitrate concentrations in the water of both wells changed analogously. The highest concentrations were recorded in April, May, and June (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.

It should be noted that seasonal fluctuations in nitrate concentrations have also been observed in the water of springs at various locations. The course of seasonal changes in concentrations is very similar to the changes in wells, but in spring water, nitrate concentrations are usually considerably lower than in well water and most often do not exceed the requirements of the drinking water hygiene standard HN 24:2003.

The author believes the following explanation is sufficiently logical: In winter, when the ground is frozen, nitrates do not migrate into groundwater. Fields are not fertilised in winter, and nitrates deposited from the atmosphere onto the ground accumulate on its surface. In spring, after the thaw, nitrates accumulated on the surface over the winter travel into the groundwater and consequently into wells. At this time, intensive agricultural work begins, along with intensive fertilisation of fields with organic and mineral fertilisers. During this period, nitrate concentrations in well water reach their maximum values. As the weather warms up further, the intensive vegetation period for plants (and various microorganisms) begins, during which nitrate nitrogen is used for protein synthesis, and nitrate concentrations in the soil and groundwater decrease rapidly.

Therefore, in the author's opinion, to check, The nitrate levels in well water should be checked in spring and early summer. If, at this time, the nitrate concentration in the water does not exceed the requirements of the hygiene standards (≤50 mg/L), it can be practically guaranteed that at other times of the year there will be even less.

And now we should discuss how to remove nitrates from well water or at least reduce their concentrations. After talking about this topic with various people, one can hear all sorts of opinions, suggestions, recommendations, and recipes, starting with the idea that wells should be covered with lids.; form a protective clay layer 1–2 metres in diameter and no less than 1 metre thick around the well, or fill the well with chlorinated lime and then pump out the water; directly remove nitrates from the well. Unfortunately, none of the methods mentioned here reduce nitrate concentrations in well water. After all, a well is not a warehouse where nitrates collect and accumulate. Nitrate concentrations in well water are the same as in the surrounding groundwater that supplies the well. If it were possible to reduce nitrate concentrations in the groundwater supplying the well, then nitrate concentrations in the well water itself would automatically decrease to the same level. How to do this? A sanitary zone of a certain size should be established around the well (how exactly the size), I have to say, but in the author's opinion, it's quite a lot), where intensive farming wouldn't take place, where there wouldn't be barns with manure piles stored next to them and similar. In other words, the well should be located away from farm buildings and intensively cultivated lands. However, this is not convenient, and therefore wells are usually located as close to the houses as possible. On the other hand, if a well is installed away from one's own property, it is still unclear, nitrate levels will decrease. After all, it's not clear, Along which paths does groundwater travel. It is possible that groundwater, fertilised by nitrates from your neighbours finding their way into your well.

A guaranteed way to get rid of water with a high nitrate content is to install a deeper well. However, this will lead to other problems: the well water will most likely be ferruginous, and it may contain increased concentrations of manganese and ammonia. In this case, you will again need to install water treatment facilities.

The only guaranteed method of nitrate removal is filtering water at the correct rate through an anion exchange filter.it  filler, for example in the form of chloride (R–R₃NCl) feebly basic anion exchanger. These anion exchangers are good at removing NO₃^–Right, so₄^2– and many other anions:

R–NH₃Cl + NO₃^– → R–NH₃NO₃ + Cl^–,

2 R–NH₃Cl + SO₄^2– → (R–NH₃)₂So₄ + 2Cl^–.

We see that the filtrate increases equivalently in Cl^– ion concentration, i.e. y., removed from water 1 mekv. NO₃^– Are so₄^2– Jonoh (62 mg NO₃^– 48 mg of arsenic₄^2–), filtrate 1 mekv. (35.5 mg) increases Cl^– concentration. It is easy to calculate that removing 250 mg (4.03 meq) of NO from the water₃^–  jonų, į filtratą pereina 4,03 mekv. (143 mg) Cl^–  When the ion-exchange capacity of the anion exchanger is exhausted, weakly basic R–NH₃The anions in edible salt can be regenerated, i.e. y. Sodium chloride, tirpalu:

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

R–NH₃NO₃ + Cl^– → R–NH₃Cl + NO₃^–  linear equation

The concentration of nitrites in natural water is usually very low due to their instability (they are equally easily reduced to ammonium ions and oxidised to nitrate ions). In clean water, nitrites are difficult to detect as they are found in thousandths of a milligram per litre. They are found in slightly higher quantities at the end of the vegetation period when organic matter is present. Nitrites are an intermediate part of the nitrification process.

In the Analytical Chemistry Laboratory of the Department of Chemistry and Bioengineering at VGTU, studies were conducted on nitrate removal using a weakly basic anion exchanger. A 4 cm diameter glass column was loaded with 100 g of a Japanese-manufactured weakly basic anion exchanger in chloride form, R–NH₃The filter was used with water from the Vilnius Antaviliai water source, which meets hygiene standards, and was treated with NaNO₃ contains 107 mg/L NO₃^–  ion concentration. Filtration rate ~5 m/h. Filtration was carried out until the anion exchanger’s capacity was completely exhausted and in the filtrate NO₃^– The ion concentration returned to the initial level of 107 mg/L. The results of this study are presented in Figure 2. The study showed, Under these conditions, it is possible to remove nitrates from 40 litres of water. When filtering more water, the nitrate concentration in the filtrate begins to increase and returns to its initial level after filtering 60 litres of water.

After regeneration of the anion exchanger, the test was repeated. Almost analogous results were obtained during the second test and are therefore not presented here.

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

Having completed the second study, the anionite was regenerated not with NaCl, but with NaHCO₃(baking soda) solution, i.e. y. in place of chloride form of anionite R–NH₃Cl was obtained as a hydrocarbonate form anionite R–NH₃HCO₃. Filtering water through R–NH₃HCO₃forms an anion at the site of Cl in an exchange reaction^– Jonas attended HCO₃^– ions, which also passed into the filtrate. So, in the filtered water, not Cl increases^–, the HCO₃^– ion concentration. And that's fine, because HCO₃^– minerals give water a good taste, and their concentration is not regulated by hygiene standards. The results of this study are presented in Figure 3.

Fig. 3. Nitrate removal using a bicarbonate-form (R-NH3HCO3) anion exchanger

Comparing the data presented in Figures 2 and 3, it can be seen that the removal of nitrates using two different forms of anion exchangers yielded almost identical results.