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SECTION III: THE EFFECT OF BIOSOLIDS ON WATER QUALITY

3.1 Introduction
3.2 Nutrient Loading
3.3 Organic Compounds and Trace Metals
3.4 Summary

3.1 Introduction
Biosolids are managed by regulation to prevent any degradation of water quality. This is because biosolids have the potential to affect water quality through the leaching of at least two general kinds of contaminants: essential plant nutrients and trace metals. Ironically, these are the same components that were removed from waste water to clean up discharges. The plant nutrients nitrogen and phosphorous, when added to water, can cause algae blooms that may lead to a degradation of quality. Excess nitrogen in drinking water is a potential health hazard and the US EPA has established maximum concentration limits for consumption of nitrate and nitrite. Trace metals in water are of concern only when they are mobile and occur in amounts above safe concentrations.

The US EPA and Maine DEP have definitive standards for selected compounds in drinking water, but the ambient standards for groundwater are less clear. The ambient standards are more subjective, depending upon uses. The standard for groundwater of highest quality (GW-A) is specified as being potable and suitable for public water supplies. This implies that water quality must meet state and federal quality guidelines established in the Safe Drinking Water Act. The secondary standard (GW-B) states only that the water be suitable for other uses. Clearly, the groundwater below agricultural fields would not be used solely for public water supplies. The lack of a numerical water quality standard for GW-B generates confusion about the potential effects of biosolids on water quality. The default comparison may be with the drinking water standards, an inappropriate comparison. The impact analysis is made more difficult by the presence of other agricultural chemicals associated with manures and chemical fertilizers. A better method to index impact is to compare changes in nutrients and metals in water before and after land applications.

The goal of all land application programs is to add nutrients equal to crop requirements and to prevent over-applications that could lead to a potential loss of excess nutrients (USEPA, 2002). Sewage sludges in Maine contain, on average, more than 4 per cent nitrogen. The average nitrate plus nitrite concentration is less than 0.2 per cent and average ammonium is less than 0.6 per cent. Nitrogen as nitrate or ammonia is very water soluble and it is readily available to plants. Movement of ammonia is slower than for nitrate or nitrite because it can be adsorbed onto clay particles in soil. Being soluble, these forms of nitrogen can be removed to varying degrees in surface flow, or by transport down into ground water. Nitrate in ground water is a ubiquitous problem in agricultural areas (Kellog et al. 2000; Nolan, 2001). The Maine regulations are built on the assumption that most of the nitrogen in the Maine sewage sludges is bound in organic matter (~80%) and it is not immediately plant available. The rate at which the organic matter decomposes (mineralizes) and releases nitrogen is important for reducing water quality impacts compared to inorganic fertilizers. A slow release of nutrients provides tangible benefits to crops that need nitrogen in steady doses. In general, nutrient release from biosolids is slower than for chemical fertilizers or green manures.

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3.2 Nutrient Loading
Biosolids have been land applied successfully for many years because they have fertilizer value (Hall and Williams, 1984). Studies of nitrogen loss from field applications report that only 20 per cent of nitrate is lost after several rain events (McLeod and Hegg, 1984). It appears that most biosolids have a small amount of soluble nitrate that is released at initial application and then the slow decay of organic matter releases nitrogen in quantities that is quickly scavenged by crops (Chaney, 1990; Gilmour et al., 2000). At a monitored biosolids spreading site in Colorado, nitrate in groundwater was found to have no net change; modest increases or decreases in concentrations over time balanced out (Stevens et al., 2003). In New Hampshire, Estes and Zhao (1996) determined that biosolids applied to cropland had a minimal effect on groundwater quality because of the slow nitrogen release. McDowell and Chestnut (2002) studied nitrogen loading at a topsoil manufacturing site where biosolids were used. The only effect on groundwater quality was detected near biosolids stockpiling locations. In Pennsylvania, Shober et al. (2002) found that long-term biosolids application to cropland increased soil nitrogen, implying that loss of nitrogen by leaching was slight.

Higher amounts of plant-available nitrogen in soil are beneficial to increase fertility for crops. Biosolids release nitrogen more slowly than chemical fertilizers, so nutrient flushing is less of a significant concern (Pierzynski, 1994). An extreme case of biosolids use at high-application rates, at 1.5 to 5 times the agronomic rates, at a gravel pit reclamation site, caused a quick flush of nitrate at 50 mg/L into ground water followed by a lower but steady input of 2 mg/L (Daniels et al., 2002). Gravel pits have very porous soils and water movement can be relatively fast. In agronomic applications of biosolids, excessive nitrogen addition is neither allowed nor good farming practice.

If nitrogen from biosolids is not to affect groundwater quality, the agronomic demand must be matched to the plant-available nitrogen (Kellog et al., 2000; USEPA, 2003). Unlike manure or chemical fertilizers, biosolids can only be applied in accordance with a written nutrient management plan. There has been a considerable effort expended to estimate the appropriate loading rate for initial nitrogen utilization and subsequent releases of nitrogen during mineralization. The amount of nitrogen available to plants will be specific to each type of biosolids and the mineralization rates will be controlled by site specific conditions (Gilmour and Skinner, 1999; Gilmour et al., 2000). The mineralization rate may have half-lives ranging from hours to thousands of days (Overcash, 2004; Overcash et al., 2005). Mineralization half life is the amount of time needed for half of the starting material to be converted. Estimating the amount of nitrogen that will become available to plants is further complicated by the loss of some nitrogen as ammonia gas (Pierzynski and Gehl, 2004). If nitrogen is to be managed on a fine scale, the land application process will need to have accurate loading calculations based upon soil chemistry, crops, and biosolids. If application rates are too high, excess available-nitrogen will be lost. For example, data from forested sites indicated that nitrate loss to surface waters occurs after biosolids are applied (increased over background by a factor of 2), but ammonia export appeared to be constant (Grey and Henry, 1998). A nitrogen mass balance was not reported in the study so nitrogen loss may not be due solely to nitrogen applied in biosolids.

It is important for the nitrogen loading to match the character of a particular biosolids. In Maine, as elsewhere, the problem of nitrogen mineralization has been managed by the DEP using fixed mineralization rates (Chapter 419, Appendix A). The mineralization rate is based upon the type of sewage sludge, previous site applications, and mode of use (topdressed or incorporated). The Maine rules use a set amount of organic nitrogen that is mineralized over several years as shown in Table IV. Reference guidelines for Maine are based on Best Management Practices for Biosolids developed by the University of New Hampshire Cooperative Extension (Boub et al., 1995) and the US EPA (1983 and 1994). The formula involves calculating the available nitrogen, and making allowances for volatilization loss of ammonia if topdressed, and then correcting for the nitrogen added by previous applications. This is a common method for calculating loading rates.

TABLE IV. Mineralization of Organic Nitrogen From Sewage Sludge. Values are per cent mineralized from initial application.

Nitrogen mineralization rates determine how much nitrogen becomes soluble and plant available. Crop demand varies during the growing season and the timing of application is important in order to match the release of nitrogen with the uptake by plants. Rodriguez et al. (2003) reported on biosolids providing adequate nitrogen for maize, and mineralization provided 35 per cent of the nitrogen needed during the following year. Biosolids mineralization rates were found to be 20 to 50 per cent in a 36-week greenhouse study, consistent with Maine’s rules (Adegbidi and Briggs, 2003). An analysis of mineralization studies found that the biosolids application rate, biosolids C:N ratio, and temperature were the master variables (Er et al., 2004). These studies reflect the importance of characterizing the applied biosolids and utilization site conditions to manage nitrogen for crops.

One extreme example of nitrogen loss can occur when biosolids are stockpiled prior to land spreading. In 2002 and 2003, research was conducted in Maine to measure the loss of nitrogen from Class B biosolids stockpiles (Peckenham, 2004). Stockpiles are a much more concentrated source compared to spread biosolids and Class B biosolids are expected to have a larger moisture content than Class A. The stockpile experiment used plastic-lined cells to collect the liquid running over or through stockpiles. Even though leachate may contain an elevated concentration of nitrogen measured as total Kjeldahl nitrogen (TKN), loadings were dependent upon leachate flow rates. Loadings were calculated to be between 0.008 and 0.028 kilograms TKN/meter3/day (0.013 to 0.047 pounds TKN/yard3/day) in the footprint of the stockpile.

The loading of TKN gradually increased over the first month of stockpiling and reached a relative maximum at six to eight weeks. Loadings decreased markedly after two months because leachate flow decreased, even though concentrations of TKN in the leachate increased. Although biosolids can show elevated concentrations of nutrients and metals in leachate or run-off from a stockpile, they also have a large capacity to retain moisture and reduce run-off compared to soil (Glanville et al., 2004).

The loss of nitrogen from an unlined stockpile can have an impact on soil and groundwater below the footprint. For example, assume a field received a delivery of 100 cubic meters of biosolids for land-spreading and this stockpile sat for 30 days. Based on the nitrogen fluxes from the stockpile experiment, between 24 and 84 kilograms of nitrogen (as TKN) would be leached from the pile. Most of the nitrogen is in the form of ammonia and this may become converted to nitrate in the soil. This is enough nitrogen as nitrate-N for 0.4 to 1.3 acres of hay, but being concentrated in one small area could be lost to deeper soils or ground water. This represents a concentration of nitrate to ground water beneath the stockpile in the range of 240 to 840 mg/L. This concentration is consistent with the data from New Hampshire where McDowell and Chestnut (2002) found mean concentrations of nitrate in groundwater wells below stockpile sites approaching 60 mg/L and soil solutions had concentrations of 100 to 800 mg/L nitrate.

There is no absolute method to compare nitrogen species concentrations in biosolids with concentrations in groundwater (Oertel and Nicklow, 2003). This is because the process of nitrification (conversion from ammonia to nitrate) is mediated by microbes and rates depend upon soil conditions, as is denitrification (conversion from nitrate to nitrogen gas). However, high nitrogen loadings, such as under stockpiles, may increase nitrate-nitrogen in groundwater. Similar nitrogen enrichment has been reported for manure lagoons (Gooddy et al., 2002).

Biosolids contain phosphorous, but much of it is believed to be contained in sparingly soluble forms (Coker and Carlton-Smith, 1986; Elliot et al., 2002). According to Brandt et al. (2004) biosolids contain water-extractable phosphorous, but in concentrations far below chemical fertilizers or manures (USEPA, 2003). They report phosphorous concentrations that range from 0.5 to 14 per cent of the total mass. The limited mobility of phosphorous combined with its varying content means that biosolids should be managed for phosphorous on a case-by-case basis (Maguire et al. 2000). This conclusion was substantiated by field studies over sandy soil (worst-case scenario) that found no significant changes of phosphorus concentrations in groundwater after sludge applications (Shepherd and Withers, 2001). The evidence from agricultural regions is that nutrients - from any source - are prone to be exported in streams draining fields after excessive and multiple applications (Pyke et al., 2003; USEPA, 2003; Richards et al., 2004).

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3.3 Organic Compounds and Trace Metals
There are two important points to be considered in relation to organic compounds in biosolids- one has to do with the nature of the compound and the other is how organic compounds interact with metals. A few of the organic compounds detected in biosolids are biologically active (act as hormones) or may be suspected of having toxic effects. For instance, pharmaceuticals that enter into soil and water have been shown to affect plant growth in laboratory studies conducted at high concentrations (Jjemba, 2002). In general, organic compounds that end up in sewage solids are sparingly soluble (hydrophobic). This physical attribute acts to keep these compounds from dissolving back into water. This means that the organic compounds are not likely to end up moving into water. However, organic matter can undergo chemical or biological processes that can change the solubilities of daughter compounds.

The second aspect of organic compounds is that they may have the ability to bind with metals. Stable organic compounds can serve as a repository of metals that keeps them out of solution and otherwise immobile. The long-term stability of organic compounds that bind metals controls the release of many trace metals. The types of wastewater treatment and biosolids formed, Class A or B, will determine the characteristics of the organic constituents; factors that may control metal mobility as organic matter decays (Stacey et al., 2001). Antoniadis and Alloway (2002) determined that the leachability of cadmium, nickel, and zinc were strongly affected by organic matter derived from the parent biosolids to the point that enhanced transport caused by soluble organic matter doubled the distance these metals were leached through a soil column.

Several studies have investigated the fate of specific organic compounds known to be from the land application of biosolids (Chaney et al., 1996). Wang et al. (1995) tested a site that received 25 applications of sludge over 20 years. They determined that 90 percent of the chlorobenzene applied was gone and 10 percent was detectable as a residual in the soil. Loss from the soil was believed to be by volatilization and not leaching to ground water. Loss from the soil was related to solubility as defined by the octanol-water coefficient. Organic compounds that have high octanol-water coefficients are less likely to enter into the ground water. Wilds et al. (1991) found that polynuclear aromatic hydrocarbons (PAHs) in biosolids persisted in soils for many years (half life 2 to 9 years). The key control on how much organic material could leach into groundwater is the rate of mineralization (Jones and Evans, 2004). Some studies find specific compounds mineralize slowly: Plasticizers (Madsen et al, 1999; Lindequist et al., 1999) and Detergents (LaGuardia et al., 2001); while others found rapid rates: Steroids (Mansell and Drewes, 2004; Snyder et al., 2004; Topp and Colucci, 2004).

These studies suggest that overall, organic matter in biosolids is relatively long-lasting. In addition, organic compounds may bind with metals and keep them immobile. However, some fraction of the organic matter in biosolids, along with some metals can be transported in ground- and surface waters (Gooddy et al., 2002; Pyke et al., 2003; Peckenham et al., 2004). Existing data are insufficient to support strong conclusions about risks to groundwater. Some connections between land uses and water quality are likely to exist because of the effects of long-term agricultural practices (chemicals, manures, and biosolids). Richards et al. (2004) detected associations between organic matter and metals such as sodium, copper, lead, and molybdenum in both soil percolates and the baseflow of nearby streams.

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3.4 Summary
Biosolids contain water soluble compounds that could affect water quality. Nutrients, organic carbon, and some metals can leach from biosolids. It is important to stress that biosolids are mainly derived from the least water-soluble components of the waste stream. As biosolids age and decompose, all of the components are either consumed by biota or transferred to the surrounding media (soil or water or air). The use of good agricultural practices, including soil erosion control measures, minimizes the impact of biosolids and other nutrient sources on water quality (e.g. Mostaghimi et al., 2001). The risks posed to surface and ground waters by spreading biosolids are small when appropriate setbacks are utilized (Chaney et al., 1996; USEPA, 2003). Uncovered stockpiles on bare ground will leach small volumes of concentrated liquid that can affect groundwater with leachate containing elevated concentrations of nitrogen and trace metals.

The NRC (2002) study recommended performing multi-pathway risk analyses that would include water. In addition, the study identified the need for better monitoring and assessment of biosolids utilization. The Biosolids Summit (Dixon and Field, 2004) also stressed many of these same recommendations as well as the need for new protocols to characterize the fate and transport of chemicals from soil into water. Following a summary of the potential benefits and deficits on water quality from using biosolids as a fertilizer.

Potential Benefits:

• Required separation distances from surface water and biosolids protect water quality.
• The thickness of soils and absorption onto soil particles protects groundwater below fields approved for land application of Class B biosolids.
• Plant nutrients in biosolids are released slowly and are readily consumed by plants.
• Metals contained in biosolids are retained by organic matter and minerals in near-neutral soils.

Potential Risks:

• Nutrients from biosolids stockpiles can be leached to groundwater or be too concentration for plant uptake.
• Soluble metals from biosolids may be transported to groundwater.
• Plants can incorporate potentially toxic metals from soil solutions.
• Long-term management of soil pH is needed to minimize metal loss.

Following is a relative assessment of how Maine’s rules protect soil quality.

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