| Forside | | Indhold | | Forrige | | Næste |

Rehabilitation of Small Water Supply Facilities

Summary and conclusions

A growing number of smaller water supplies are reported to have poor water quality and are requested to make improvements. In many cases the safest and easiest way is to be connected for instance to the public water supply, but in the open country this is not always an obvious possibility. If the distance to the nearest public water supply is long, the resulting transport period may affect the quality, and the costs of connecting can grow unreasonably high. In such cases it is an option to rehabilitate the water supply facility.

This project deals with the rehabilitation of 11 facilities with water quality problems, establishing the technical and economic guidelines in relation t°Contaminations by nitrate and pesticides and to bacteriology.

The selected facilities are characterised by their construction, reservoir and contamination, and information was obtained regarding water quality, supplemented with new analyses.

After renovation of the facilities, sampling was carried out after approximately two weeks, as a minimum, and again after one year, to assess both immediate and long-term effects. Among the analytical parameters, focus was on problem parameters.

Based on water analyses made after renovation, the results of facility renovation are summarised in the table below.

Summary

The level of costs for renovation varies from DKK 3,000 to DKK 50,000 per facility. Deep wells and borings, which are re-bored or extended with new filter constructions, are the most costly. At boring DGU no. 160.653 the renovation costs are about equal with the costs of a new boring.

In the table below the costs (rounded figures) are specified for the individual facility.

Renovation costs

* Water quality requirements for selected analysis parameters

The tables below present results of the renovation process shown as function of construction and contamination types, as well as reservoir type. Figures with % refer to the percentage number of facilities (distributed on construction type, respectively reservoir type and contamination type), where the renovation process has involved improved conditions.

Results in relation t°Construction type

Results in relation to reservoir type

The overall success rate for the renovation process is summarised in the table below.

Overall results

Water quality has been improved in 7 of the 11 facilities, while in 4 facilities clear changes or improvements of the water quality after renovation were not registered. In some instances, however, the bacteriological conditions have deteriorated. This applies to facilities that were not disinfected (by chlorination) immediately after renovation. Several of the facilities were chlorinated since then, and water samples show that conditions are now satisfactory.

The renovation has not had an effect on the facilities where the nitrate concentration was too high, since the nitrate concentration was practically unaltered. This could indicate problems with nitrate in the groundwater reservoir (regionally or locally).

In a single boring (168.1374), however, the nitrate concentration was almost halved from app. 28 mg/l to app. 13 mg/l (average values). In this specific incidence it was proven that the problem was mainly percolation of surface water. Still, the phosphorus concentration in the same boring remained almost unaltered around 0.12 mg/l. In boring 158.853, where there were problems with too high phosphorus concentration next to a high nitrate concentration, the renovation on the other hand involved a decrease in phosphorus from app. 1 mg/l to app. 0.11 mg/l, whereas the nitrate concentration in the same boring remained the same or was increasing, and considerably above the limit value. For these two borings it therefore seems unlikely that there should be a correlation between the source of phosphorus and nitrate contamination.

Conclusively, the renovation of the three nitrate-impacted borings, 160.453, 158.853 and 142.874, has not had the desired effect, i.e. to obtain compliance with drinking water requirements. For the latter, though, the transgression is very small (50-53 mg/l) and will probably not release an immediate demand for improvements of the water quality, although perhaps a closer surveillance over the next years. All facilities are, however, offered a discontinuation.

Regarding pesticides it seems that the renovation process has involved an improvement of conditions for most facilities (5 out of 6). In one boring (142.596) the pesticide concentration remained above the limit values, which can probably be ascribed to a local reservoir contamination. The boring is therefore offered a discontinuation. Renovation activities in connection with pesticide contamination, such as improvement of well constructions or replacement of borehole (surface) sealing, has a high success rate, which indicates that pesticide contamination in the renovated facilities was related to infiltration of contaminated surface water or contaminated groundwater from secondary reservoirs. It is not possible to establish after the relatively short time (app. one year after renovation) whether the demonstrated positive effect of the implemented improvements is sustainable.

Renovation was highly effective for all facilities with poor bacteriological conditions. The high success rate is ascribed to the fact that the renovation activities aimed at preventing percolation of contaminated surface water, which is thought to be the source of contamination.

In general there is no unambiguous correlation between the level of renovation costs and the effect (improvement of water quality) of renovation. In most instances the costs of renovation are still relatively low compared to establishing a new boring, which would typically be the alternative, if private or public waterworks are located far away.

Generally speaking, investigations before renovation prove that most facilities are in poor condition as regards installations, including pumps, pipes and water treatment facility. In most cases renovation of the boring (or well) must be supplemented by a thorough renovation/replacement of installations.

For facilities with bacteriological problems, renovation will lead to a very likely improvement of conditions at relatively low costs. It is recommended that borings/wells always be disinfected after renovation, and in some instances it may be necessary t°Carry out disinfection (using chlorination) several times.

Facilities with high nitrate concentrations should only be renovated to the extent that it can be firmly proven that problems are caused by contaminated surface water. The investigation shows that it is highly unlikely that renovation of the facilities will involve a significantly lower nitrate concentration.

Based on the modest scope of the investigation regarding facilities with pesticide contamination it is not safe t°Conclude that a significant improvement can be obtained in all cases, but considering the relatively small costs of renovation activities, it is an obvious possibility for improvement.

It is recommended to discontinue one boring (142.596), due to a high pesticide concentration that has largely been unaltered or increasing since the investigation and renovation process. Three borings (160.453, 158.853 and 142.874) are offered discontinuation because of nitrate contents above limit values at the latest round of analyses.

| Forside | | Indhold | | Forrige | | Næste | | Top |

Version 1.0 Marts 2004, © Miljøstyrelsen.