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SECTION II: THE EFFECT OF BIOSOLIDS ON SOIL QUALITY AND CROPS

2.1 Introduction
2.2 Agronomic Value
2.3 Metal Mobility
2.4 Trace Metal Uptake By Plants
2.5 Metal Accumulation in Soil
2.6 Organic Compounds
2.7 Summary

2.1 Introduction
The following summary emphasizes recent studies that are based upon several years of elapsed time for field and laboratory studies. The land-application of biosolids is generally condoned because of its nutrient content and soil-conditioning properties. This has been repeatedly demonstrated and a short review is provided in Section 2.2. Here the emphasis will be on the associated risks and the management of those risks. Particular attention is placed on evaluating the effects of heavy metals in biosolids. These heavy metals come from human wastes, corrosion of metal pipes in plumbing systems, and since the advent of pre-treatment technologies, declining inputs from industrial sources. Therefore, biosolids will have some measurable content of heavy metals that could pose a health risk via multiple pathways if directly ingested in unsafe quantities. Under certain conditions the long-term addition of heavy metals from manures, biosolids, and other soil amendments can accumulate in soils with possible environmental consequences. These trace metals may enter the food chain via uptake by plants used for food or fodder, or via grazing animals. Finally, these heavy metals may be leached from soils and could affect water quality. Persistent organic compounds can exhibit similar types of behaviors in crops, soil, and water.

Biosolids are land applied primarily for their plant-nutrient value provided by nitrogen, the primary controlling nutrient. Maine’s rules also have provisions to monitor phosphorous loadings in sensitive watersheds. In addition, biosolids are screened on a regular basis for trace metal content. This screening is mandated to limit the amount of metal that is added to the soil in both single applications and cumulative from multiple applications at the same location. Trace organic compounds in biosolids are regulated in Maine, as are dioxins. The Maine DEP has established screening concentrations limits in 06-096 CMR, Chapter 418, for 579 hazardous substances that include inorganic and organic compounds. As a result of this substantial pre-screening requirement, more is known about the chemical content of biosolids than any other soil amendment.

In Maine, for nearly all biosolids applied to land, trace metal loading is not a limiting factor for biosolids utilization. As described in Section 1.6, almost all biosolids in Maine have metal concentrations well below the maximum allowable. The fate of trace metals in biosolids is of interest to scientists trying to understand whether metals could enter the food chain or just remain inert in soil. This has been the focus of research for many years and there are numerous relevant publications cited in this paper. The scientific interpretation of these studies can be summarized into two generalizations:

1. metal loading by biosolids is strictly regulated by state and federal regulations and it is not a significant concern because there is a soil-plant barrier that limits metals uptake by plants and even if metals accumulate in soils, the general food chain is protected, and
2. metals accumulate in the soil, bound to solid matter, but certain metals could become mobile or plant-available over time as they are released from binding sites.

In general, there is ample evidence for the agronomic value of biosolids as a soil amendment or fertilizer. The Maine regulations use nutritive chemistry of biosolids as the basis for setting the amount of biosolids that may per applied to land. This nutritive content of biosolids is orders of magnitude greater (1,000 to 10,000 times) than the quantities of trace metals and persistent organic compounds. The risks posed by these other components are the crux of some objections to land applying biosolids. In particular, do these trace components accumulate in crops, accumulate in the soil, or leach into water supplies?

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2.2 Agronomic Value
Some of the earlier studies of land applying sewage sludge supported the procedure because the nitrogen content was considered to be free fertilizer. Sewage sludge was treated as being directly analogous to animal manure. Biosolids, like animal manures, are rich in both organic matter and nitrogen, making them desirable amendments for agricultural fields (Hall and Williams, 1984; Stukenberg et al., 1993; Kellog et al., 2000). The agronomic value of such amendments was confirmed with field testing that reported increases in soil humic matter, cation exchange capacity, and nitrogen after repeated applications (Stadelman and Furrer, 1985; Estes and Buob, 2001; Shober et al., 2002). Biosolids are not a total replacement of fertilizers and although they have fertilizer value, they do not provide a balance of nutrients and need to be managed as part of a farm nutrient management plan. In a well-managed setting with appropriate timing of applications, biosolids provide a significant nutrient gain for farmers (Pierzynski, 1994; Oberle and Keeney, 1994). Beyond any fertilizer value the addition of biosolids to soil lowers bulk density, and increases porosity, moisture retention, and organic carbon (Lindsay and Logan, 1998). The application of biosolids to an apple orchard improved soil physical properties and increased crop yield (Neilsen et al., 2003). Roka et al. (2004) determined that limed biosolids had an agronomic value of $5.90 per ton. There is a substantial body of research available on the agronomic value of biosolids that is not reviewed here (e.g. Larson et al., 1994). The 1993 conference sponsored by the Soil Science Society of America (Clapp et al., 1994) and the National Research Council (1996) report on sludge and food crops contain many papers and citations relevant to beneficial use. Biosolids compare favorably with manures (see Figure 5) and provide similar fertilizing properties (Moss et al., 2002). As a point of comparison, the trace metal and organic chemical composition of manures and fertilizers are not regulated as strictly as are biosolids.

2.3 Metal Mobility
An early focus of research was on the presence of trace metals and the leaching of metals from soil. As a result, applied research was directed toward metals in biosolids, much more so than organic chemicals and pathogens. Current regulations draw heavily from this research. It should be stressed that some of the sewage sludges studied more than 20 years ago were of lower quality than what is allowed for land application under current regulations (Issac and Boothroyd, 1996).

One of the first steps was to determine if metals in sludges (pre-biosolids) were mobile. Gerritse et al. (1982) determined that some metals were mobile, the three most mobile being manganese, strontium, and antimony; all other metals were sorbed onto soil particles at pH >6. It is assumed that plants can uptake only the soluble metals. Andersson (1984) found that sewage sludges had a different soluble metal fraction with decreasing solubilities starting with the most soluble: cadmium > zinc > nickel > cobalt > copper> manganese > lead > chromium. When the same sludges were composted the order of solubility changed to: zinc > cadmium > manganese > lead >nickel > cobalt > copper> chromium. Composting changed the way some metals were bound in the sludge. According to Andersson (1984) and Speir et al. (2003), although several metals are soluble, zinc is most likely to be incorporated by plants. Zinc is not a great concern because it has a relatively low toxicity. Plant uptake of cadmium has been a focus of additional research because of its potential toxicity in the food chain.

Metal solubility does not necessarily mean that transport into plants or groundwater is occurring. Soluble metals may be loosely held onto the surface of soil particles. For example, cadmium concentrations in soil following seven years of sludge application (initial Cd loading = 3.2 kg/ha) at the Askov site in Denmark were detectable only in the first 25 centimeters and none was detected in the shallow groundwater (Larsen, 1984). This and other early studies indicate that metals in biosolids may be mobile, but for most metals plant-uptake or transport downward in soil is limited.

The work completed during the 1980’s was useful to set the direction for following research. In particular, the fate of metals in soils was not thoroughly understood. Some questions remained:

• Do any metals in biosolids enter the food chain via plant uptake?
• Does plant uptake of metals from soil follow a linear dose-response relationship?
• Do metals from biosolids that are bound in organic matter become mobile over time?
• Can metals be leached into groundwater?

These questions continue to drive some of the metals research that continues today.

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2.4 Trace Metal Uptake By Plants
Some trace metals in biosolids may function as micro-nutrients in plants and animals. Micronutrient metals include: copper, iron, manganese, molybdenum, and zinc. However, the same, or other, trace metals may accumulate up the food chain via plants and thus could present a health risk to grazing animals and humans. Metals must occur in relatively high availabilities in soils to represent a risk of transfer to plants. The uptake of cadmium and zinc by plants at some biosolids utilization sites has been documented (Stukenberg et al., 1993). Cadmium has been studied more closely because is appears to be the most toxic heavy metal that is likely to accumulate in plants and so serves as a useful indicator, while the imbalance of copper and zinc may affect health in some grazing animals (Muchi et al., 1987; Chaney, 1994; Smith, 1994).

Excessive concentrations of some metals will inhibit crop growth, or even kill crops, and this effect is called phytotoxicity. Even the chemical form of the metal is important. Field and laboratory studies by Chaney (1994) showed that experiments using metal salts generated different results from actual biosolids. This difference is attributed to using metal salts in the experiments that are highly soluble and release the metals easily into solutions. This is in contrast to the strong metal-organic complexes that form in the biosolids and that are much less soluble, and thus the metals are less available. The understanding of the cause-and-effect relationships and dosing experiments are further complicated by the mechanisms that control how metals are incorporated into plant tissues.

The essential problem is discovering if metals stay in the soil, are incorporated into plants (bioavailability), or are leached deep into soil or ground water (leachability). The public health risks come from metals entering the food chain or dissolving into drinking water. The research shows this problem to be difficult to answer.

Plant uptake of metals, regardless of source, is not a linear dose-response, but a complex curve with an uptake plateau at elevated soil concentrations. This means that small increases in metal concentrations in soils having low background concentrations may cause small increases of metal concentrations in plants. When background concentrations are greater, increasing the content of metal in soil will eventually cause no additional increase in metal concentrations in plants. In a sense, plants can filter out metals and limit how much enters via their roots.

The metal-uptake-plateau theory is the basis for Chaney (1994) and Chaney et al. (2004) to state that plants can act as a bio-barrier for metals. Other interpretations of similar data exist, including an exception for cadmium (Dudka and Miller, 1999). Brown et al. (1996) have a contrary interpretation of soil-plant relationships and determined that uptake follows a log-linear relationship. This model defines the metal loading to soil as the dose and the plant uptake is a function of the logarithm of the dose. They suggest that crops can be indexed by their relative metal uptake. The soil-metal-plant relationships defy simple conclusions. For instance, Logan et al. (1997) presented a different interpretation of similar experimental results; a linear dose-response of metal uptake in crops for up to two years after biosolids application, followed by a plateau. Plant uptake was linear with biosolids application rates for the period studied. Clearly, these different experiments did not replicate the identical conditions; another possibility is that the data reflect different parts of similar dose-response curves.

Organic-matter and minerals in soil and biosolids mediate complex interactions that minimize the uptake of metals such as cadmium. This indicates that for most high-quality biosolids, plant uptake of cadmium is not a concern (Chaney, 1996; Brown et al., 1998; Speir et al., 2003). The authors imply that total metal loading may not be the limiting factor in assessing risks because the solubility, or plant-availability, of the metals was over-estimated in the regulations.

A comparison of these different soil-plant relationships is shown in Figure 6. The key difference is that in the linear model, plants take up more metal directly in proportion to the amount added above some threshold value. In the plateau model, plant uptake is similar to the linear model at low loadings. However, the plateau model includes an absolute maximum capacity for metal uptake. Metal loading to soil beyond a certain value will not increase plant uptake. The two types of metal-uptake relationships plotted in Figure 6 are offset for clarity. At low metal loadings, the plant uptake rates are similar for both models (both curves have the same slope). In the plateau model, when plant uptake reaches a specific limit, uptake slows or stops. The plateau model implies that as soils accumulate metals, plants can actually dilute metals entering the food chain (Chaney, 1996). It is possible for the differences between the two models to be very slight below the plateau concentration. In such a situation, the data of Chaney et al. (2004) may cover the whole plateau curve, while Logan et al. (1997) assessed the early and linear region, and Brown et al. (1998) collected data on the portion of the curve approaching the plateau. The protection offered by the plateau is important, but plant-specific constants are lacking.

The plant-availability of trace metals from biosolids is controlled by the chemistries of both the biosolids and the soils; principally defined by the organic matter of the biosolids and the sorptive capacity of the soil (Basta, 2004; Basta et al., 2005). The pH of the biosolids and soils is the chemical variable with the single greatest control over the availability of trace metals (Heckman et al., 1987; Mulchi et al., 1987; Chaney, 1994; Basta and Sloan, 1999; Basta et al., 2005). These factors must be considered when interpreting trace metal uptake experiments. The plateau effect could be a function of soil pH conditions that cause trace metals to be held by organic matter or adsorbed to minerals. At soil pH >6, the total metal content can be high, while the plant-available concentration is much smaller (e.g. McBride et al., 2004). This may explain why Chang et al. (1997) analyzed 15 years of land-application data and found neither a plateau nor a re-release of metals. This is possible if plant-available metal loadings were at low concentrations and plant uptake was in the linear range.

FIGURE 6. Comparison of metal loading and plant uptake.
Figure 6 demonstrates how plants respond to different metal loadings to soil, some increase in proportion to concentration while other exhibit a maximum amount that can be incorporated.

Although many of these studies attribute the availability of trace metals solely to soil chemistry, there are other possible controls on metal uptake by plants. Trace metal uptake is modulated by plant physiology and it is the plants themselves that control the presence or absence of a plateau (Hamon et al., 1999; Maisonnave et al., 2001). It is well established that plant species have different nutrient needs, including functional differences relative to metal uptake. This suggests three master variables interact to define the availability of trace metals in soil and the rate of uptake by plants:

1. Soil chemistry, especially soil pH,
2. Biosolids composition, and
3. Type of plant.

Even though potentially toxic metals such as cadmium may enter into plant crops, the actual concentrations in plants appear to be below harmful limits (Brown, et al., 1998; O’Connor and McDowell, 1999). Estes and Buob (2001) found no differences in metal content in corn grown with or without biosolids. The US EPA risk assessment used referenced studies to establish maximum loading rates for metals in biosolids and is rated as being very conservative (Ryan and Chaney, 1993; Logan et a;., 1999). The term conservative means that the greatest plant uptake rates were used to complete the risk assessment and real risks are likely to be orders of magnitude lower. However, McBride (1998) believes that the model used by the EPA employed uptake coefficients that were too low because the data were generated using optimal soil acidities (pH >6) studies and most natural soils in the northeastern United States are more acidic. Under more acidic conditions the metals would be more mobile and bioavailable and uptake rates could be higher. Good agricultural practices require soil pH to be managed close to pH 6.

McBride (1998), McBride et al. (2004) and several other researchers (Chaney, 1994 and 2004; Smith, 1994; Speir et al., 2003) raise the important issue of soil pH management on fields receiving biosolids. In addition to pH, the presence of soil amendments containing high concentrations of iron, manganese, or phosphorous reduce the bioavailability of heavy metals such as lead, zinc, and cadmium (Brown et al., 2003; Brown, et al., 2004). This means that a fraction of the trace metals are likely bound in forms (organic or oxy-hydroxide complexes) that may be less sensitive to soil pH. In Maine, where soils are naturally acidic, the re-mobilization of trace metals is possible if soils are not managed to maintain optimal soil pH. Maintenance of soil to near neutral pH must be a long-term objective for land application sites.

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2.5 Metal Accumulation in Soil
If plants only take up a small fraction of the metals added to soils from biosolids, then the metals could be accumulating in the soil. The fate of metals from biosolids is directly addressed for long-term management of utilization sites under Maine law. Accumulation of metals is the reason for having annual and cumulative loading limits. It has been recognized for many years that metals from sewage sludge accumulate soils having pH values of 6 to 8 units (Gerritse et al., 1982). It is a well-understood geochemical principle that under oxidizing and non-acidic conditions, most metals tend to form oxides or hydroxides that are sparingly soluble. Compounds of iron hydroxide or phosphorus can also bind trace metals. The exceptions are molybdenum, vanadium, uranium, and selenium (Levinson, 1974), all of which occur in low concentrations in Maine’s biosolids.

If metals accumulate in oxidized soils at a nearly neutral pH, then it should be possible to account for the trace metals added from biosolids and removed by plants. Several studies of long-term utilization sites have attempted to perform mass balances for metals. Stukenberg et al. (1993) reported that metal accumulation was mostly limited to the top 12 inches of soil. McBride et al. (1997) re-tested a utilization site 15 years after application and found elements added from biosolids were lost: 100 per cent for sodium, sulfur, calcium, and strontium; 40 per cent for zinc and copper; and less than 30 per cent for cadmium and phosphorous. Water-soluble copper, nickel, and zinc concentrations in the treated soil were ten times greater than the control soil. Surprisingly, these soluble metals had not been leached from the soil, but soil solutions in the treated soil had higher concentrations than a control field (McBride et al., 1999). Berti and Jacobs (1998) evaluated metal accumulation at a site with 10 years of biosolids application history. They found that metal recovery ranged from 45 to 155 per cent of loading; how metals could accumulate post-application is difficult to explain but probably reflects natural variability. In their mass balance they found that losses accounted for 20 per cent of all the chromium and nickel, 30 per cent of the cadmium, and 40 per cent of the lead and copper; zinc was essentially unchanged (zero loss). At another site with repeated applications of biosolids over a period of 11 years, Babarick et al. (1998) reported statistically significant metal accumulations relative to control soils. The metal loading was confined to within the plow layer (< 30 cm) with slight evidence of metals moving deeper into the soil. Estes and Buob (2001) reported no significant increases in trace metals in soils due to biosolids applications at study sites in New Hampshire. Sloan et al. (1998) found very high metal recovery rates (100 % ±) at the Rosemount site after 16 years of biosolids spreading. Long-term monitoring, 23 years post-application, was conducted by Sloan et al. (2000) and they found that after the first two years cadmium and zinc remained nearly unchanged. They indicated that the organic matter holding the metals is mineralizing very slowly and is expected to persist for more than 100 years. Some of the variation in mass loading can be attributed to uncertainties in actual loading rates due to analytical limitations.

Molybdenum has gained additional scrutiny recently because of its phytotoxicity and unique geochemical properties (O’Connor and McDowell, 1999). Field studies conducted by Brinton and O’Connor (2003) suggest that the presence of iron and aluminum greatly reduce the bioavailability and mobility of molybdenum. However, as with other trace metals, the soil pH remains the most important control.

The scientific literature presents data that appear to be contradictory regarding metal accumulation and loss. These differences are probably due to the use of different biosolids in different soils in different climate regimes. The importance of high soil pH as an inhibitor to metal mobility (Gerritse et al., 1982; Basta and Sloan, 1999; Speir et al., 2003; Basta, 2004; Basta et al., 2005) and the metal-binding capacity of organic matter (Sloan et al., 1998; McGrath et al., 2000; DeVolder et al., 2003) have been amply demonstrated. More recent work has demonstrated the importance of metal-binding oxides in soils (Basta et al., 2004 and 2005). The chemical controls are in turn a function of the soil that results from the effects of climate on the local geology. Climate drives soil thermal regimes and moisture content; two factors that will affect biosolids decomposition rate and the release of trace metals. Another source of variability is the composition of the original sludge. Stehouwer et al. (2000) found that the metal content of sewage sludges in Pennsylvania could vary by factors of 10 to 50 per cent. They also reported that the quality of the sludges improved in the 1990’s to have less variability and, in many cases, lower metal content.

The fate of metals in soil at biosolids utilization sites is difficult to predict without site-specific data, hence Maine’s regulations err on the side of being conservative and assume greater mobility. This in turn means that acceptable metal loadings have a margin-of-error built in. In general, a small portion (<1%) of trace metals are bioavailable and incorporated into crops (Berti and Jacobs, 1998; Shober et al., 2002; Chaney, 2004). A much larger portion remains (> 50%) in the soil either bound to organic matter or to soil particles (Bell et al., 1991; Basta, 2004 and 2005). Some fraction of a few metals, like selenium and mercury, may be lost via volatilization (Capon et al., 1984). McBride et al. (2004) report that the bioavailability of zinc and copper can be elevated for many years. Although most of the research finds metal accumulation within 30 centimeters of the surface, up to 30 per cent of some trace metals may be transported to greater depths, including to ground water (Camobreco et al., 1996; McBride et al., 1997; Richards et al., 1998). The compositions of the starting materials (biosolids and soil) are key to initial bioavailability of metals and their subsequent mobility (Richards et al., 2000; Merrington et al., 2003; Speir et al., 2003). Soil and plant analyses in Maine show no evidence of significant plant uptake of metals (Houtman et al., 1995).

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2.6 Organic Compounds
There are numerous organic compounds in biosolids that come from diverse sources such as the organic wastes (fecal matter) and products that are used at home that end up in the waste stream, including pharmaceuticals and personal care products (Daughton and Ternes, 1999). Many of these compounds are water soluble (i.e. polar) and partition into water rather than solids during the waste treatment process (Heberer et al., 2002). Thus relatively few pharmaceuticals accumulate in sewage sludge. In addition, some organic compounds are destroyed during the treatment process. Thus, chemical principles predict that few organic compounds can persist through the waste treatment and biosolids processes to enter the food chain (Wild and Jones, 1992). This principle is supported by a recent survey of 39 wastewater treatment plant sludges in Canada that found no organic contaminants in environmentally relevant concentrations except for seemingly ubiquitous heavy petroleum hydrocarbons and some PAH compounds (Bright and Healy, 2003). Some of these organic compounds come from plastic pipes or stormwater runoff from roadways. Much of the organic matter, including trace organic chemicals, will be mineralized (decayed). Only a very small number of organic compounds do not decay over human-relevant timescales. As with the fate of trace metals, the research does not indicate a large potential threat from organic compounds in most biosolids. The trace organic compounds detected usually occur in concentrations below accepted regulatory action limits (for instance, Maine DEP Risk Assessment for Direct Contact, and Chapter 405).

There is growing attention being paid to biological active chemicals in wastes. These are pharmaceuticals and personal care products (PPCP) that have human and veterinary uses or otherwise affect the endocrine system. Because many PPCPs are used for therapeutic reasons, they can pass through humans or animals unchanged and end up in the waste stream as active compounds, even at very low concentrations (Daughton and Ternes, 1999). These compounds were detected in 80 percent of 139 streams sampled by the U.S. Geological Survey in 1999 and 2000 (Kolpin et al., 2002). The most frequently detected compounds in this stream survey were: coprostanol, cholesterol, N,N-diethyltoluamide, caffeine, triclosan, tri(2-chloroethyl)phosphate and 4-nonylphenol. Little is known of the fate of these compounds in the environment and the synergistic effects when combined in exposed populations. Available studies suggest that these compounds may not be significantly accumulated in biosolids. Many of the compounds are water soluble and partition into water rather than solids during the waste treatment process (Heberer et al., 2002).

Ongreth and Khan (2004) report that only three pharmaceuticals of 24 examined exhibited significant concentration into sewage sludge (gemfibrozil, eryhthromycin, and carbamazeprine). All of these compounds were detectable in the low ppb range and other pharmaceuticals may partition into sewage sludge in much lower concentrations (Khan and Ongreth, 2002). These studies suggest that the risks posed by pharmaceuticals in sewage sludge are small because they occur in concentrations well below their intended therapeutic range.

Several researchers have found that some organic compounds of potential concern do not persist in biosolids, or at land-application sites. Wang et al. (1995) evaluated chlorobenzene in soils from the Woburn, England site that had 25 sludge applications between 1942 and 1961. They determined that only 10 per cent of the added chlorobenzene remained in the soil. Analysis of archived soils suggested that most of the chlorobenzene was lost through volatilization. Alexander (2000) presents an argument that the bioavailability of organic compounds naturally decline over time. Compounds that exhibit this effect are some polynuclear aromatic hydrocarbons (PAHs)- naphthalene, phenathrene, anthracene, fluoranthene, pyrene; the herbicide atrazine, and 4-nitrophenol.

Bright and Healey (2003) reported that many toxic organic compounds that were once found in sludges just are not detected in modern sludges. Chlorophenols, poly-chlorinated biphenyls (PCBs) and chlorinated pesticides are typically below detection limits. However, some aromatic compounds, p-cresol and phenol; polynuclear aromatic hydrocarbons (PAHs) such as phenanthrene, pyrene, naphthalene; and long-chained hydrocarbons in the C19 to C34 range were detected. Overall the loading of PAHs to soil is far below the level of toxicological concern. Topp and Colucci (2004) provided evidence that estrogen-like compounds (or endocrine disrupters) dissipate rapidly from soil, often in hours to days. This dissipation is hastened by oxidation, a process favored by surface application of biosolids.

It needs to be stressed that trace organic compounds are part of the biosolids organic matter. The distribution coefficient, K_d is a measure of the ratio of the compound that binds onto the organic mass relative to the concentration in the surrounding liquid. These trace organic compounds are naturally and strongly sorbed to the whole organic mass with K_d’s much greater than 10⁶ (Jones and Evans, 2004). In this case the concentration on the solid is more than one million times more concentrated than in water. These authors all suggest that trace organic compounds either disappear, degrade, or become biologically irrelevant within months of contact with the soil.

Some researchers have found that some persistent organic compounds are associated with biosolids. Wilds et al. (1991) indicate that PAHs can be detectable for many years in soils. They have determined half-lives for PAH compounds in the range of 2 to 9 years. Wang et al. (1995) stated that although much chlorobenzene dissipates, 10 per cent remains 42 years after application. Listed organic compounds are found only occasionally in Maine’s biosolids. A common trace organic contaminant is the phthalate ester di-(2-ethylhexyl) phthalate (DEHP) that adds flexibility to innumerable plastics, including the common PVC drain pipes. Considered a non-toxic compound, its metabolites may have estrogenic effects and it may persist for many years (Madsen et al., 1999). The available research has not indicated any effect on human health. Another commonly encountered trace organic compound is alkylphenolethoxylate. This compound degrades to octylphenol, nonylphenol, nonylphenol mono-ethoxylate, and nonylphenol di-ethoxylate in concentrations sufficient to be a potential risk in surface waters, although there is no evidence that biosolids are a significant source (LaGuardia et al., 2001). Hale et al. (2002) have detected the fire retardants bromo-diphenyl ethers that are similar to polybrominated biphenyls. The occurrence rate is higher for urbanized areas and may affect urban sewage sludge quality.

The methods of analysis and toxicity risk-assessment tend to overstate risks (conservative models). Like some trace metals that may not be present in biologically significant concentrations, many organic compounds occur in biosolids at low concentrations. Data for Maine’s biosolids show that the types of organic compounds discussed here are relatively rare.

Organic matter, in its most general sense, in biosolids is persistent and only slowly mineralizes, including the trace organic compounds (Jones and Evans, 2004). This is a desirable quality that improves soil quality. The compound specific data available to evaluate the risks posed by these compounds is limited, but the majority of the compounds are likely to be benign (Daughton and Ternes, 1999; Kester et al., 2004 and 2005). The risks posed by many organic compounds in waste streams appear to be small because they either: 1) do not partition into sewage solids, 2) are degraded during the treatment process, or 3) decay when exposed to sunlight. Data from Maine indicates that the content of hazardous organic compounds in biosolids is generally quite small and restricted to a small number of compounds.

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2.7 Summary
Biosolids are complex mixtures of organic matter that have agronomic value, as well as containing trace metals, such as cadmium, zinc, and copper. Survey data of application sites suggest that the benefits greatly exceed the deficits (Houtman et al., 1995; Shober et al., 2002). Biosolids also present some risk to grazing animals and humans due to plant incorporation of chemical constituents or exposure from accidental ingestion of solids. The latter route requires very close physical contact. The risk posed to crops fertilized with biosolids is difficult to determine because soils are heterogeneous and crop responses are subject to numerous environmental variables. There is some measurable transfer of metals from biosolids in soil to certain crops, but the amount of transfer up the food chain appears to be small. Managing soil pH to be circum-neutral minimizes the loss of metals to plant uptake or leaching to groundwater. Data collected in Maine suggest that risks posed by trace metals at biosolids utilizations sites are negligible.

The NRC (2002) study recommended performing an updated survey of biosolids quality to determine how metal content has changed and to inventory other potentially hazardous substances. In addition, the study identified the need for field-based studies to evaluate the effectiveness of setbacks and operational oversight. This includes performance standards for the production of Class A and B biosolids. The Biosolids Summit (Dixon and Field, 2004) reiterated many of these same recommendations as well as the need for new protocols to characterize the fate and transport of chemicals of concern.

Following is a summary of the benefits and risks of using biosolids as a fertilizer.

Potential Benefits:

• Inexpensive source of nitrogen.
• Source of trace nutrients and phosphorous.
• Biosolids increase soil organic matter and improve moisture regulation.
• Concentrations of heavy metals in Maine’s biosolids are well below the US EPA exceptional quality standard.
• Transfer of metals to food crops is limited.
• Organic matter in biosolids binds with metals and lowers their bioavailability.

Potential Risks:

• Biosolids contain some trace metals of concern, but nearly all in Maine are below regulatory risk thresholds.
• A small fraction of nutrients and metals may leach from biosolids into groundwater.
• Added metals may persist in soils for decades and slowly become bioavailable.
• Soil pH needs to be managed over long time periods to minimize metal losses.

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

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