Metal Content of Grasses at Land Treatment Sites

Metal

Melbourne, Australia Started 1896 Sampled 1972

Control Site Measurement

Locations (Concentrations (mg/kg)

Fresno, California Started 1907 Sampled 1973

Manteca, California Started 1961 Sampled 1973

Livermore, California Started 1964 Sampled 1973

Metal

Cadmium

0.77

0.89

0.9

1.6

0.3

Copper

6.5

12.0

16.0

13.0

10.0

Nickel

2.7

4.9

5.0

45.0

2.0

Lead

2.5

2.5

13.0

15.0

10.0

Zinc

50.0

63.0

93.0

161.0

103.0

indicated that essentially all of the metals applied could be accounted for in the top 50 cm (20 in.) of the sandy soil, and over 95% were contained within the top 15 cm (6 in.) (Reed, 1979).

Although the metal concentrations in typical wastewaters is low, concerns have been expressed regarding long-term accumulation in the soil that might then affect the future agricultural potential of the site. Work by Hinesly and others as reported by Reed (1979) seems to indicate that most of the metals retained over a long period in the soil are in forms that are not readily available to most vegetation. The plants will respond to the metals applied during the current growing season but are not significantly affected by previous accumulations in the soil. The data in Table 3.16 demonstrate the same relationship. At Melbourne, Australia, after 76 years of application of raw sewage, the cadmium concentration in the grass was just slightly higher than in the grass on the control site, which received no wastewater. The other locations are newer systems in California, where the cadmium content is the same order of magnitude as measured at Melbourne, suggesting that the vegetation in all these locations is responding to the metals applied during the current growing season and not to prior soil accumulation. The significantly higher lead in the three California sites as compared to Melbourne is believed to be due to motor vehicle exhaust from adjacent highways.

Metals do not pose a threat to groundwater aquifers, even at the very high hydraulic loadings used in rapid-infiltration systems. Experience at Hollister, California, demonstrates that the concentration of cadmium in the shallow groundwater beneath the site is not significantly different than normal offsite groundwater quality (Pound and Crites, 1979). After 33 years of operation at this site, the accumulation of metals in the soil was still below or near the low end of the range normally expected for agricultural soils. Had the site been operated in the slow rate mode, it would have taken over 150 years to apply the same volume of wastewater and contained metals.

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