Using Compost for a Safer Environment

Some composts, particularly those derived from industrial organics, may contain elevated concentrations of heavy metals and nutrients. Since these chemicals are potentially harmful to streams and aquatic life, it’s important to know whether runoff from composts that are enriched with metals or nutrients is likely to contribute to increased amounts of these pollutants in nearby streams.

As shown by statistics in table 1, the bio-solids compost tested in this study contained significantly higher concentrations of eight metals and two nutrients than any of the other composts or soils. Although the metal concentrations in the bio-solids compost were higher than in the other materials they were well below upper limits set by the USEPA for "high quality" biosolids and would not be considered to pose an environmental hazard.
With the exception of arsenic, statistical analysis shows that soils associated with the conventional treatments generally contained the lowest concentrations of nutrients and metals. Yard waste and bio-industrial composts contained higher levels of some nutrients and metals than were found in the conventional soils, but considerably less than in the bio-solids compost.

Table 1. Average nutrient and metal concentrations in composts and soils.
Element	Compost Treatments			Conventional Treatments
	Biosolids	Yard Waste	Bio-industrial	Compacted subsoil	Topsoil
	Mean Concentration (milligrams per kilogram)
Arsenic	BDL*^a	4.620^c	1.968^b	4.817^c	3.815^c
Cadmium	1.630^c	BDL^a	BDL^a	BDL^a	BDL^a
Chromium	61.687^d	9.118^b	15.985^c	9.778^a,b	8.253^a
Copper	193.573^d	21.325^b	69.458^c	6.950^a	8.733^a
Mercury	2.370^b	1.607^a,b	BDL^a	BDL^a	BDL^a
Molydenum	7.492^b	0.882^a	BDL^a	BDL^a	BDL^a
Nickel	18.743^c	9.900^b	14.680^b	11.928^a,b	8.635^a,b
Lead	70.443^d	26.085^b	59.118^c	19.658^a,b	13.715^a
Selenium	BDL^a	BDL^a	BDL^a	BDLa	BDL^a
Zinc	1,033.54^d	139.358^b	307.632^c	42.672^a	45.722^a
Nitrogen	25,560.0^d	18,962.7^c	11,758.3^b	1,070.08^a	1,391.33^a
Phosphorus	15,702.8^d	2,582.33^b	2,887.55^c	332.532^a	438.955^a
Potassium	5,951.81^c	10,906.6^d	3,269.08^b	858.030^a	746.385^a
Means within the same row with different letter designations are significantly different (p<0.05). *Value below analytical detection limit.
	Highest	Medium	Lowest

Despite the significantly higher concentrations of several metals and nutrients in the composts (particularly biosolids compost) very few of these potential water pollutants were found in runoff from compost-treated areas. Zinc, phosphorus, and potassium were the only soluble pollutants present at detectible levels in the liquid portion of the runoff samples.

With the exception of phosphorus in runoff from the biosolids compost, the total soluble mass of each of the three pollutants contained in runoff caused by a 30-minute storm was significantly lower in compost runoff than in runoff from conventionally-treated test plots (table 2). This is primarily the result of the significantly lower runoff produced by the compost blankets.
Runoff from vegetated test plots contained lower total masses of soluble phosphorus and potassium than was found in the runoff from un-vegetated plots. Again, the total mass of pollutants was much lower in runoff from test plots treated with compost than from conventionally-treated test plots.

Table 2. Soluble mass of nutrients and metals in un-vegetated plot runoff from 30-minute high-intensity (100 mm/hr) rainstorm.
Element	Compost Treatments			Conventional Treatments
	Biosolids	Yard Waste	Bio-industrial	Compacted subsoil	Topsoil
	Geometric Mean (milligrams)
Zinc	<0.01^a	<0.01^a	<0.01^a	0.15^b	016^b
Phosphorus	0.17^b,c	<0.01^a	0.01^a,b	1.38^c	0.76^c
Potassium	1.08^a	0.09^a	0.29^a	49.55^b	18.01^b
Means within the same row with different letter designations are significantly different (p<0.05).
	Highest	Medium	Lowest

Only 5 metals and 3 nutrients were detected in the eroded solids contained in runoff from the test plots.
As was the case for the soluble pollutants, the total mass of adsorbed pollutants carried by eroded particles in runoff caused by a 30-minute high intensity storm was significantly lower for compost-treated areas than for test plots treated conventionally (table 3).
Data presented in table 3 show absorbed pollutants found in runoff from from un-vegetated plots. Similar trends among treatments were exhibited by runoff samples collected from vegetated test plots.

Table 3. Adsorbed mass of nutrients and metals in un-vegetated plot runoff from 30-minute high-intensity (100 mm/hr) rainstorm.
Element	Compost Treatments			Conventional Treatments
	Biosolids	Yard Waste	Bio-industrial	Compacted subsoil	Topsoil
	Geometric Mean (milligrams)
Chromium	0.01^b	<0.01^a	<0.01^b	0.92^c	0.76^c
Copper	0.02^b	<0.01^a	0.01^b	1.03^c	0.66^c
Nickel	<0.01^b	<0.01^a	<0.01^b	0.96^c	0.67^c
Lead	0.01^b	<0.01^a	<0.01^b	1.82^c	0.95^c
Zinc	0.10^b	<0.01^a	0.03^b	6.55^c	3.99^c
Nitrogen	0.47^b	<0.01^a	0.09^a,b	266.65^c	211.87^c
Phosphorus	0.45^b	<0.01^a	0.09^a,b	36.47^c	29.07^c
Potassium	0.17^b	<0.01^a	0.09^a,b	103.94^c	71.57^c
Means within the same row with different letter designations are significantly different (p<0.05).
	Highest	Medium	Lowest

The excellent retention of both soluble and adsorbed nutrients and metals by the relatively pollutant–rich composts is the combined result of low runoff (see Project Results – Runoff Quantity) and low erosion (see Project Results – Erosion Control) exhibited by these materials.