Chlorination byproduct potential in components of the nyc municipal water supply

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One illustration of some potential water quality issues for treated water derived from different components of the NYC municipal supply can be

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Figure 7.7. Laboratory chlorination experiments: total trihalomethane concentration vs time, with raw water from Rondout Reservoir (unfiltered); Hudson River at Chelsea (unfiltered and filtered).

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Figure 7.7. Laboratory chlorination experiments: total trihalomethane concentration vs time, with raw water from Rondout Reservoir (unfiltered); Hudson River at Chelsea (unfiltered and filtered).

gained by considering results of laboratory chlorination experiments using raw water from two of those components (Bopp et al., 1990). Total trihalomethane (TTHMs) concentrations (the most abundant trihalomethane treatment byproduct is chloroform), following chlorination of raw water derived from Rondout Reservoir, increased from 0 to 33 parts per billion (ppb) in 100 hours (Fig. 7.7). Parallel experiments, using unfiltered and filtered Hudson River water from Chelsea, at the point of withdrawal for the NYC system, resulted in TTHM concentrations of 191 and 165 ppb, respectively, after 100 hours, a factor of about five to six times greater (Fig. 7.7). Thus if Rondout water and Hudson River water were blended in proportions of 90 percent and 10 percent, the resultant TTHM concentration in laboratory experiments would be about 40 percent higher than if the raw water supply were 100 percent Rondout water. Although these simple experiments should not be considered as accurate simulations of THM levels within the actual distribution system following chlorination, they do provide insights about relative potential chlorination byproduct levels for different raw water components of the NYC municipal supply. One tentative conclusion is that addition of only modest proportions of Hudson River water to the treated flows into the delivery system could potentially have a substantial impact on chlorination byproducts in the blended water. Similar experiments illustrate the TTHM potential of Croton System water (Bopp et al., 1990; unpublished data). Laboratory chlorination of samples from three reservoirs produced an average of 173 ppb

TTHM, after seventy-two hours. A sample of mixed Catskill and Delaware System water under the same experimental conditions produced 73 ppb. Furthermore, THMs should also be considered as indicators of formation of other chlorination byproducts, such as di- and trichloroacetic acids.

Dissolved organic carbon (DOC) fromboth natural and anthropogenic sources provides precursor compounds that lead to formation of chlorination byproducts in treated water supplies (including di-and trichloroacetic acids as well as THMs). The three Croton System samples had DOC levels of 4.29, 4.53, and 4.65 mg l-1 and yielded 153, 173 and 193 ppb TTHM, respectively. DOC concentrations of eight samples from Chelsea and five samples from other Hudson locations (1985) averaged 5.45 mg l-1, while five samples from Delaware System reservoirs averaged 2.31 mgl-1 in [DOC] (Bopp et al., 1990). Surveys of DOC during 1988-89 in the tidal Hudson suggest lower values during spring high discharge, but no appreciable variation along the axis of the river, and mean values of about 4 mg l-1 (Findlay, Pace, and Lints, 1991). There appears to be sufficient DOC in the tidal freshwater Hudson during most periods to result in formation of substantially higher concentrations of chlorination byproducts than from most Catskill area surface waters.

Samples of Hudson River water were collected from the NYC distribution system immediately after chlorine had been added at Chelsea on two separate occasions during the drought emergency of 1985 (Bopp et al., 1990) when pumping of water from the Hudson occurred from July 10th to December 11th. These stored samples from the NYC system had elevated TTHM concentrations, quite comparable to those from our laboratory chlorination experiments. The highest levels observed (Fig. 7.8) were 236 ppb (10/29/85) and 189 ppb (11/26/85), well above the current permissible level of 80 ppb for TTHMs.

Serious consideration has previously been given to the concept of operation of some reservoirs in the Catskill Mountains for both municipal water storage and pumped-storage hydroelectricity generation. This would permit additional supplies of electricity during peak demand hours by transfer through turbines of water from higher elevation to lower elevation reservoirs. During low demand pe-

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Figure 7.8. Total trihalomethanes vs time, in samples collected from the NYC municipal system on two dates, of treated (chlorinated) water withdrawn from the Hudson at Chelsea and stored in the laboratory.

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Figure 7.8. Total trihalomethanes vs time, in samples collected from the NYC municipal system on two dates, of treated (chlorinated) water withdrawn from the Hudson at Chelsea and stored in the laboratory.

riods (night), water would be pumped from lower to higher elevation reservoirs with electricity from base load facilities elsewhere (nuclear and fossil fuel generating stations). Construction of a new, higher elevation, reservoir near Schoharie Reservoir (Catskill System) was proposed for a number ofyears. Such apair of coupled reservoirs is in operation at Blenheim-Gilboa on Schoharie Creek, just downstream of the NYC municipal supply watershed. DOC concentrations in water from these two pumped storage reservoirs averaged about 35 percent higher than in Schoharie Reservoir, although the sources of runoff were very similar in both cases (Simpson, Bopp, and Deck, 1981;Bopp et al., 1990). Since DOC concentrations are one of the major factors influencing chlorination byproduct levels in treated water, conjunctive operation of municipal water storage reservoirs withpumped-storagegen-eration of electricity could potentially have negative consequences on some aspects of NYC water quality following treatment. Additional DOC in raw water supplies, from whatever source, shouldprob-ably be minimized whenever possible.

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