OVERVIEW OF THE ENVIRONMENTAL IMPACT OF FLUORIDATION C.E. Herold1 and M. van Veelen2 1Umfula Wempilo Consulting, PO Box 98578, Sloane Park, 2152. Tel:+27(0)11 463-5203. Fax +27(0)11 706-8524. E-mail: [email protected] 2BKS (Pty) Ltd. ABSTRACT The environmental impact of fluoridation on representative downstream water bodies is presented. An assessment system was developed to derive preliminary management objectives against which to compare receiving water fluoride concentrations. A simple methodology was developed to estimate the change in downstream median and peak fluoride concentrations. More detailed assessments carried out by Rand Water were used for the Vaal River system and a portion of the upper Crocodile River system. Fluoridation to 0.7 mg/l could lead to substantial problems in the strategic Vaal Barrage and downstream Vaal River, Crocodile River, Waterval River and Sand River systems. Problems can be anticipated in the Modder River upstream and downstream of Krugersdrift Dam. Insignificant impacts were indicated in the Msunduzi River, Berg River and Buffalo River systems. The limitations of the methodology and key knowledge gaps are highlighted. More detailed evaluations taking account of monthly hydrological fluctuations, dam storage and system operating rules need to be carried out to optimise the fluoridation targets in the identified key problem areas. The coarse overview should be extended to embrace other potential problem areas. The long-term effect on irrigated lands, groundwater and irrigation return flow quality warrants attention. INTRODUCTION The Department of National Health legislated regulations requiring water service providers to fluoridate the water supplied to up to 0.7 mg F/l. The Water Research Commission (WRC) has conducted an overview of the feasibility of using water as a vehicle for the distribution of fluoride to the South African population. Specialist teams were assembled to deal with the health, environmental, social and legal, technical and economic aspects. This paper focuses on the environmental impact on downstream water bodies after fluoridation of water supplies (1). Return flows are the main driving force controlling the environmental impact of fluoridation. The most significant impact arises in areas where effluent discharge makes a major contribution to river flow and present fluoride concentrations in the effluent are low. This is exacerbated by relatively high diffuse input from industrial or natural sources, evaporative concentration in arid and semiarid regions and cascading water use down the river system. Inland areas with substantial downstream water use are of greater concern than systems where the effluent is discharged to marine outfalls. The resources of the overview did not permit investigation of every locality. Instead effort was concentrated on a few carefully selected priority systems where significant impacts are likely to arise. These included the Vaal Barrage - Middle Vaal River, upper Crocodile River, Msunduzi River, Sand River, Modder River, Berg River, Buffalo River and Waterval River systems. Figure 1 shows the areas covered in the assessment and some additional areas that may warrant further attention. Proceedings of the 2004 Water Institute of Southern Africa (WISA) Biennial Conference 2 –6 May 2004 ISBN: 1-920-01728-3 Cape Town, South Africa Produced by: Document Transformation Technologies Organised by Event Dynamics Figure 1. Identified potential problem areas. ASSESSMENT SYSTEM A generic assessment system was developed, taking account of the requirements for domestic and agricultural use and the aquatic ecosystems. The change in concentration that may arise from fluoridation of drinking water is not expected to manifest in acute effects, but rather as chronic effects manifesting themselves after a long period of exposure. In this range the central tendency is more important than the instantaneous concentration. Hence the statistical distribution of the individual measurements over time should be used to determine the fitness for use. As water quality is not statistically normally distributed (a concentration can not be negative), non-parametric statistics are used to describe the statistical distribution of fluoride concentrations. The following parameters were used: 50th percentile: An indication of average conditions. 75th percentile: The upper limit of the inter-quartile range, which indicates the upper limit of what a user will be exposed to on average. 95th percentile: Indicative of extreme conditions of short duration. This should still fall below acute effect levels. Water quality can also not be described simply as “good” or “bad”, but as a gradual change from one to the other. The categories and descriptors that have been generally accepted are: Ideal water quality: where there is no effect even after continual prolonged use. Acceptable water quality: where only in rare instances some sensitive users may be affected after a long period of use. Tolerable water quality: where sensitive users may be affected after prolonged use. Unacceptable water quality: where there is a risk that chronic effects may occur. The SA Water Quality Guidelines (2) showed that domestic use is the most sensitive user across all categories. This finding was unaffected after consideration of the proposed ecological classes for fluoride. Table 1 shows the resulting assessment system. Table 1. Assessment system for fluoride (mg/l). Class Percentile upper limit 50% 75% 95% Ideal 0.7 0.7 0.7 Acceptable 0.7 1.0 1.0 Tolerable 1.5 1.5 1.5 Unacceptable Any other combination Even the tolerable category will have a significant risk of undesirable effects, either on humans or the aquatic ecosystem, and is therefore cause for concern. Moreover, Table 1 should be viewed as a means of categorising a water body, rather than as defining a management objective. Management Objective It would be an exercise in brinkmanship to load the system with a pollutant to raise its concentration to the very limits of the acceptable range. Responsible management requires the setting of management objectives somewhat below the limits of acceptability. This is necessary to allow for future growth, unforeseen circumstances and the coarse nature of the current evaluation. The purpose of this investigation is not to set such management objectives. However, a tripwire is required to determine if more detailed investigation is required. A 30% buffer was used for this purpose. The preliminary management targets are given in Table 2. Table 2. Preliminary management targets for fluoride (mg/l). Class Percentile upper limit 50% 75% 95% Ideal 0.5 0.5 0.5 Acceptable 0.5 0.7 0.7 Tolerable 1.05 1.05 1.05 Unacceptable Any other combination The upper limit of the acceptable level (i.e. a median of 0.5 mg/l, and a 95-percentile value of 0.7 mg/l) represent the tripwire level above which more detailed investigation is indicated. ESTIMATION OF CHANGE IN QUALITY AFTER FLUORIDATION The results of a preliminary impact assessment of fluoridation of the Rand Water supply (Herold, 2002) were used for the Vaal Barrage-Middle Vaal River and parts of the Crocodile River systems. This analysis was based on an 11-year time series of monthly effluent and river flow data ending December 2001, the current change in fluoride concentration from the raw water to effluent discharge and estimation of river losses between sampling points. While this preliminary methodology does not handle storage changes in major reservoirs well, it is considered to give an adequate initial estimate of the likely impact of fluoridation. For other areas a much cruder assessment was based on the average effluent discharge, the median river flow and 5-years of fluoride data at key points in each river system. The method is discussed below. Estimation of Median Concentration For pre-fluoridation conditions the mean and 95 percentile concentrations were obtained from the last 5-years record. Assuming no change in the fluoride load added by users, the load added to the effluent by fluoridation was estimated as the difference between the post- and pre-fluoridation average concentration multiplied by the average effluent discharge. This was then added to any increase in load calculated for upstream stations (after allowing for abstractions) and divided by the observed median flow at the downstream river station to arrive at the median increase in fluoride concentration at this point. This was then added to the observed median concentration to yield the estimated post-fluoridation concentration at the downstream river station. This methodology is intended to provide a rapid coarse evaluation of the fluoride concentration likely to arise in river reaches below effluent discharge points. Time and budgetary constraints did not permit application of the mass balance to a long time series reflecting fluctuations in runoff rate and catchment fluoride export concentration. Median conditions were chosen, since the main concern is with chronic conditions, which are governed by long-term exposure. Knowledge of the present or future effluent fluoride concentrations or of the change in fluoride concentration through use is not needed. All that is required is the increase in load brought about by fluoridation. For river stations the median flow was used since this more closely represents the typical day by day exposure (the average is too strongly influenced by short duration flood discharges). For dam stations the average is a better approximation of exposure