ASSOCIATIONS OF SOURCE WATER TYPE AND TOTAL ORGANIC CARBON CONTENT WITH THE DETECTION OF COLIFORMS IN DRINKING WATER SUPPLY ZONES
Report No FR0479

May 1994

SUMMARY

I BENEFITS

Identifying correlations between zones with detectable coliforms and source water type and total organic carbon (TOC) content would:-

  1. establish whether these factors predispose some zones to produce coliform-positive samples intermittently throughout the year compared to other zones which do not.

  2. enable the effects on zonal coliform compliance of certain operational strategies (e.g. mixing source waters) to be predicted.

  3. provide further information with which to develop predictive models for coliform concentrations within the drinking water supply

II OBJECTIVES

The objectives are to correlate coliform detection in zones with total organic carbon content and source water type.

III REASONS

Statistical analyses performed so far on the FWR contract, "Water Quality in the Distribution" (F-1702) have concentrated on zones with coliforms, and in particular on identifying differences between samples with and without coliforms. Those associations will assist in the development of models to predict coliform concentrations in different parts of the water supply network. The factors studied included colony count organism concentration, temperature and total chlorine concentration. However, no information is available, as yet, on how differences between supply zones may influence annual coliform incidence and zonal compliance. Zonal differences may be operational in nature reflecting source water type, extent of mixing, nature of water treatment processes, retention time and degree of storage in service reservoirs. In addition, waters may differ in TOC content between zones, the average TOC content being an estimate of this. Assessing whether some of these factors are linked to poor coliform compliance in zones would indicate operational treatment processes which predispose some zones to produce coliform-positive samples intermittently throughout the year. In addition, zonal influences on coliform compliance need to be established for the development of models to predict coliform concentrations. Thus, if it were shown that an association existed between surface water supplied zones and higher zonal incidences of coliforms, then models based, e.g. on chlorine and temperature, may be different for surface water supplied zones compared to zones supplied by other source water types.

IV CONCLUSIONS

Associations between source water type and zones with coliform-positive 100 ml volume samples were tested for Water Company C. More zones supplied by mixed water sources were coliform-positive than expected if no association. Also, the frequency of coliform detection in ground water supplied zones was lower than expected if no association. However, these associations were not statistically significant at 5%. Differences in the proportions of coliform-positive ground water zones (20.7%) and mixed water zones (32.0%) were approaching statistical significance at 5% (P = 0.085). Average TOC contents were statistically much lower for ground water supplied zones than for mixed and surface water-supplied zones in Water Company C. This is consistent with wh at one might expect. No difference in TOC content was detected between mixed and surface water supplied zones. Associations between average zonal TOC content and coliform-positive zones were not apparent using data from all zones pooled together in Water Company C. Separate analysis of data from zones according to source water types showed no association between coliform detection and TOC for mixed and surface water supplied zones. However, there was a possible association between lower TOC content and the detection of coliforms in ground water supplied zones (P ~ 0.07).

For Water Company H, a strong association (P = 0.003) between higher zonal average TOC content and coliform-positive zones was found. Furthermore, a similar association existed between higher trihalomethane contents and intermittent detection of coliforms in water supply zones in Water Company H. Trihalomethanes are produced by the action of chlorine on components of TOC.

V RECOMMENDATIONS

Differences are apparent between zones with and without coliform-positive samples. This analysis is of particular interest because it may determine what is special about zones which never or very infrequently register coliform-positive samples. It is recommended that further associations be assessed in other companies. Further analysis should concentrate on data for zones supplied by the same source water type.

Further work should be performed to assess whether low TOC content is an important factor in coliform incidence in groundwater supplied zones in Water Company C.

VI RESUME

Section 1 summarises the work performed so far in FWR contract F-1702 in relation to coliform compliance of water supply zones. The benefits of defining associations between zones with detectable coliforms and source water type and TOC are discussed. The statistical methods used are described in Section 2. In Section 3 results of statistical associations between source water type and coliform detection are presented for Water Company C. TOC levels in zones supplied by three types of source water are compared for Water Company C. TOC contents are compared between zones with and without coliforms for Water Companies C and H. Trihalomethane data are also investigated for Water Company H. Conclusions are made in Section 4.

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