Effect of compaction on removal efficiency of lead, copper, zinc, nitrate, and phosphate in a bioretention system: a column study

Stormwater management has become an important part of the field of water resources management. The traditional method of disposing of urban stormwater was to drain it away as quickly as possible. However, in recent years, questions concerning the impacts on the receiving waters began to arise, and in response to these concerns, some communities have elected to encourage capturing and treating stormwater runoff. This is accomplished by having a portion of the stormwater infiltrate into the soil. A bioretention system is an innovative practice of pollutant control used for this purpose. These systems combine the concepts of detention ponds and biological treatment in an attempt to provide higher overall pollutant removal. However, little is known about the overall efficiency of bioretention. The purpose of this study was to see if compaction affects the pollutant removal efficiency (PRE) of a bioretention system. Fifteen columns were constructed of 8 inch diameter PVC piping. Series One through Five had bulk densities of 1.1, 1.2, 1.4, 1.5, and 1.7 g/cm3. The contaminants examined were lead, copper, zinc, nitrate, and phosphate. Soil compacted too much or too little was less efficient at removing metals from the runoff. Soil with a bulk density of 1.4 -- 1.5 g/cm3 was more efficient at removing the metals than soil with bulk densities of 1.1, 1.2, and 1.7 g/cm3. However, compaction of the soil did not have a significant (p[less than]0.05) impact on the PREs of the metals. Also, there was no significant effect of compaction of the soil on the PREs of the nutrients. The PREs (which were usually negative) for both nitrate and phosphate improved as the compaction increased. The soil that had a bulk density of 1.7 g/cm3 had the highest PREs for both nutrients. This was an unexpected result, as most of the previous studies state that it is not a good tactic to compact the planting soil of a bioretention system as it tends to decrease the infiltration rate.M.S.Includes bibliographical references (p. 55-65)