Fate of Disinfection By-Products in the Subsurface

By Colleen Rostad
U.S. Geological Survey, P.O. Box 25046, MS 408, Denver Federal Center, Lakewood, Colorado 80225

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Abstract

Disinfection of drinking water involves adding chemicals to the source water to kill pathogens and viruses. These chemicals oxidize organic matter present and reduce odor and color of the water, but in the process produce disinfection by-products (Larson and Weber, 1994). Disinfection by-products can also be formed during wastewater disinfection. Concentrations of disinfection by-products (DBPs) in drinking water are subject to government regulations (Pontius, 1996, 1999). Prior to discharge, wastewater is heavily chlorinated and subsequently dechlorinated. Because of high dissolved organic carbon (DOC) concentrations in wastewater, high concentrations of DBPs result (Jekel and Roberts, 1980, Fujita et al, 1996). These are discharged into surface or groundwaters, where they may be diluted, volatized, or sorbed to nearby sediments. In wastewater discharged to surface waters, discharge permits usually limit free chlorine (due to sensitivity of aquatic organisms), rather than disinfection by-products.

The quantity and speciation of DBPs, such as total organic halide (TOX) and trihalomethanes (THMs) can vary not only by water quality and disinfection conditions, but also by properties of the organic molecules that comprise the DOC (Croue et al., 1999).  Natural organic matter characteristics of source waters, such as molecular weight distribution and humic content, can vary due to some geographical or source-related (surface vs. groundwater) differences (Owen et al, 1993).

Although much is known about disinfection processes and factors that influence by-product formation, less is known about their fate in the environment. Most groundwater recharge is done with chlorination-disinfected wastewaters. Studies have shown that in surface waters TOX is relatively non-volatile and conservative, whereas THMs volatilize. In groundwater assessments across the country, the most common contaminant found was chloroform (a prevalent chlorination by-product), possibly from municipal drinking water infiltration (Squillace et al., 1999). Ozone is rarely used to disinfect wastewaters, and therefore ozonated water is rarely used for recharge. Ozonation increases the biodegradability of natural organic matter, resulting in greater bacterial regrowth in ozonated waters without further biological treatment (Reckhow, 1999), which is a detriment to efficient recharge.

Effect of aquifer recharge on DBPs and their formation potential is important for eventual retrieval and potable use of the water (Bouwer, 1991, Asano, 1992; Pinholster, 1995). Groundwater recharge can be accomplished by surface spreading or direct injection. However, concerns about water quality and potential health hazards led to California issuing guidelines for groundwater recharge, recommending spreading over injection, disinfection prior to recharge, and minimization of DBPs (Crook et al, 1990). Groundwater recharge is more expensive using injection wells than infiltration basins (Bouwer, 1991).

In order to assess the effectiveness of wastewater treatment by groundwater recharge (soil aquifer treatment), two aspects are important:  the fate of DBPs themselves, and changes in the DOC, which affect the disinfection by-product formation potential upon reuse. However, extensive studies on DOC fractionation and THM formation potential (Owen et al, 1993; Croue et al, 1999; Croue et al, 2000; Hwang et al, 2000) rarely are applied to groundwaters.

Initial studies on groundwater recharge by direct injection of reclaimed municipal wastewater found that although total organic carbon and THMs decreased, the TOX showed no retardation or sorption in the aquifer (Roberts et al, 1982). Using secondary and tertiary treated wastewater for aquifer recharge, a decrease of 50 percent of DOC and 40 percent of TOX was accomplished by shallow (20-ft) soil aquifer treatment, while the THMs volatilized in the recharge ponds prior to infiltration (Amy et al, 1993). Most removal occurred within the top meter of the surface. Higher molecular weight, humic material was preferentially removed by the soil aquifer treatment (Amy et al, 1993), as has been reported for granular activated carbon (Owen et al, 1993). Soil aquifer treatment at the same site using greater depths (80-ft) reduced DOC by 92 percent and TOX by 85 percent (Wilson et al, 1995). Soil column studies with secondary effluent showed DOC removal of 56 percent for sandy loam, 48 percent for sand and 44 percent for silty sand (Quanrud et al, 1996a). While 48 percent of the DOC was removed with most removal near the surface, absorbable organic halide removal, assumed to be sorption, was only 17 percent, (Quanrud et al, 1996b).

Reclaimed water derived from secondary and tertiary treated wastewater has been discharged into spreading basins to recharge groundwater in Los Angeles County, California for 30 years. TOX and THM concentrations were determined in 1996 at the source, and down gradient with multilevel samplers located from 4 to 20 feet below the spreading basin. Elevated THMs and TOX were found below and in distant down-gradient wells (Barber et al, 1997). If TOX were sorbing out or degrading, it would decrease relative to DOC. The proportion of TOX to DOC remained consistent along the gradient sampled. There was also no decrease in THMs relative to TOX in the aquifer. Yield of TOX and THMs did not change significantly during infiltration (Leenheer et al, 2001). Therefore, precursors of disinfection byproducts in reclaimed water are not rapidly removed by soil aquifer treatment.

In twenty-three nearby production wells, TOX correlated with DOC with a linear correlation coefficient, r², of 0.61. In contrast, TOX correlated with UV absorbance at 254 nm with a much lower r² of 0.42. Specific UV (SUVA, ratio of UV absorbance to DOC) was much lower than other natural waters (Amy et al, 1987). On four of the production wells, total THM yield (ug THM formed per mg DOC in source) were low, compared to similar literature values (Rathbun, 1996). Non-purgeable TOX yield was consistently lower than other yields reported (Rathbun, 1996). The very low specific UV and low yields of THM and TOX indicate that the DOC is much less reactive with chlorine than commonly found in surface waters. Low reactivity would be expected from reclaimed water, which has been chlorinated and dechlorinated prior to release. Conclusions from these formation-potential studies were that the aquifer recharge was not contributing TOX or THM formation potential to this water during recharge.

An aquifer in northern Florida was investigated where the surface water connects to groundwater through limestone karst. Karst features, such as sinkholes, provide direct pathways for the flow of surface water into the unconfined aquifer. The surface water sources are referred to as blackwater streams due to the high concentrations of tannins and lignins. Formation potentials of the surface and groundwater were quite different. The DOC was fractionated to determine which specific fractions were the most reactive (Rostad et al, 2000b).

Two groundwater samples were dominated by hydrophobic acids, with minor amounts of the other fractions. Two surface water samples were dominated by hydrophobic acids and colloids, which made up about 75% of the DOC, and significant amounts of hydrophilic acids and tannins. About a third of the organic carbon was in the colloid phase. Surface waters were much more complex than the groundwaters, even though they are directly connected, and linked to each other. There was a dramatic decrease in DOC between the surface and ground waters, and the DOC in these waters are quite different chemically. TOX and THM production for the surface and ground waters indicated that the colloid and hydrophobic acid fractions were more reactive than the other fractions per milligram DOC.

SUVA is a commonly used indicator of formation potential based on UV absorbance (Minear and Amy, 1996). In comparison of TOX yield as a function of SUVA, at all four sites the hydrophilic acids and bases had greater yield than SUVA predicted. Hydrophobic acids, and hydrophobic neutrals were less reactive than SUVA predicted. When THM yield was compared to SUVA, at all four sites the THM formation potential was even more consistent in the trends of (1) hydrophilic acids and bases had greater yield than SUVA predicted; (2) Hydrophobic Acids, and hydrophobic neutrals were less reactive than SUVA predicted. Fractions of the DOC reacted to produce different disinfection by-product yields. The overall effect of limestone aquifer on DOC was that the high concentrations of DOC present in the surface waters were either sorbed or precipitated out by the limestone karst during infiltration.

Formation potentials were determined for 24 hours and for 7 days in waters entering and exiting a demonstration constructed wetland near Phoenix, Arizona (Rostad, 2000b). In 24 hours, the DBPs were dominated by TOX. Although this water is chlorinated and dechlorinated prior to discharge to the wetland, reactivity of the remaining DOC was greater than expected. Dissolved organic carbon produced by the wetland was highly reactive in producing THMs. After 7 days, the formation of THMs dominated over TOX. In seven days the water exiting the wetland formed slightly less THMs than TOX, although the amounts were greater. Because formation of THMs increases after the first 24 hours, utilities prefer to keep residence time down to 24 hours in distribution systems. Compared to the incoming wastewater, the constructed wetland increased the formation potential of the water both for TOX and THMs.

In general, under soil aquifer treatment conditions, DOC and THMs decreased, but TOX showed less attenuation. Most removal occurred near the surface. Unlike constructed wetland treatment, formation potential of TOX and THMs was not increased by soil aquifer treatment.

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