CICEET Progress Report for the period 2/01/03 through 7/31/03

Project Title: Engineering Bioretention for Treatment of Storm Water Runoff
Principal Investigator(s): Allen P. Davis

Accomplishments
Scheduled Tasks:
The work for the sixth six-month period of the project focused on Task 1-- Mechanistic Experiments and Task 3 -- Data Analysis.

Progress on Tasks
The objective of this part of the study is not only to test the effectiveness of bioretention for retaining phosphorus (P) from runoff, but also to evaluate the leaching potential of retained P to ground or surface water and P availability for plant growth.

A second twelve-column experimental set using the same media (as described in report V) has been completed to evaluate bioretention performance during repetitive storm events. Before and after the experiments, media samples were collected from different depths in the column for P extractions. The media P investigations include TP test, environmental soil tests (water soluble P and calcium chloride extractable P) and agronomic soil tests (Mehlich I extractable P and Mehlich III extractable P tests).

Water-soluble P was determined by mixing 2.5 g of soil with 25 mL of deionized water for 1 hr. Calcium chloride extractable P was analyzed by shaking 5 g of soil with 20 mL of 0.01 M CaCl₂ for 24 hrs. Mehlich I extractable P was determined by shaking 2.5 g of each media with 10 mL of Mehlich I reagent (0.05 M HCl+ 0.0125 M H₂SO₄) for 5 mins (Sims and Heckendorn, 1991). Mehlich III extractable P was determined by shaking 2.5 g of each media with 25 mL of Mehlich III reagent (0.2 N CH₃COOH+ 0.25 N NH₄NO₃+ 0.015 N NH₄F+ 0.013 N HNO₃+ 0.001 M EDTA) for 15 mins (Mehlich, 1984). Finally, P in all extractants was analyzed using the Murphy and Riley (1962) method.

A paper entitled "Evaluation of Bioretention for Treatment of Urban Storm Water Runoff" was presented at the World Water and Environmental Resource Congress 2003, Philadelphia, PA, June 22, 2003. This conference was sponsored by the American Society of Civil Engineers.

Also, an abstract "Multiple-Event Study of Bioretention for Treatment of Urban Storm Water Runoff" has been selected as a Podium Presentation and will be presented at the 7th International Conference- Diffuse Pollution and Basin Management, which will be held during August 17 to 23, 2003 in Dublin, Ireland. This conference is sponsored by the International Water Association.

Two manuscripts entitled "Evaluation and Modification of Bioretention Media for Treatment of Urban Storm Water Runoff" and "Long-Term Evaluation of Bioretention for Treatment of Urban Storm Water Runoff- Phosphorus Removal" are in preparation for submission to referred journals.

Difficulties Encountered
No major difficulties have been encountered.

Anticipated Success in Meeting Project Objectives in Scheduled Project Period
All experimental work for this project was completed by July 31. The final report is being compiled and it is expected that all project objectives will be met by November 30, 2003.

Results
Repetitive Experiments
A total of sixteen repetitions employing the same media, which the interval between each repetition is 5-14 days, were completed to test the long-term performance of bioretention, including the infiltration rate and pollutant removals. The results for runoff infiltration rate are presented in Figure 1. Based on the results, runoff infiltration rate gradually decreased from 0.51 to 0.16 cm/min throughout the first fourteen tests. Suspended solids appeared to clog the bioretention surface after completing 14 repetitions. The 15th repetition was started 14 days after the 14th repetition, which was twice as long as the period between the first 14 repetitions, to simulate the field condition without rain for a longer period. The infiltration rate increased from 0.16 to 0.26 cm/min after the longer dry period. In order to test remediation effectiveness on the surface clogging, the top 5-cm of medium was scraped and filled with new original medium. In response, the runoff infiltration rate recovered to the same level as the initial (<0.50 cm/min).

Compared with the previous repetitive column (RP1), this column (RP2) initially resulted in a higher runoff infiltration rate (0.51 cm/min) than RP1 (0.36 cm/min). However, the runoff infiltration rate through RP2 gradually decreased to 0.16 cm/min after 14 repetitions due to surface clogging. This problem was improved after a longer drying period (15th repetition) and 5-cm surface media replacement (16th repetition). In RP1, mulch filtered most of the TSS in the runoff, preventing it from clogging.

TP removal results are presented in Figure 2. TP was nearly all removed in the first 7 repetitions (41 days). After this period, the removal efficiency of TP gradually decreased and finally reached 56% in the 14th repetition. Similar to the runoff infiltration rate, TP removal efficiency increased from 56 to 67% after 14 days drying. This change might be due to some P initially adsorbed on the media surface finally diffusing into the media matrix, releasing new surface for more P to adsorb. The removal efficiency of TP was further increased to 89% after replacing the top 5-cm medium. Because of the effectiveness of this newly-restored top 5-cm medium for TP removal, the importance of surface bioretention media was confirmed.

In order to investigate the effect of media depth on TP removal, media at different depths were taken to test for the P level before and after repetitions. The results are summarized in Figure 3. The P level in all media increased; from 231 to 485 and 478 mg-P/kg- media for top and lower mixture media, from 53 to 133 and 148 mg-P/kg- media for top and lower sand media, and from 100 to 460 mg-P/kg- media for bottom soil media. Apparently, the soil captured larger amounts of P (61 mg-P/kg) than sand (average= 3.75 mg-P/kg) and mixed media (average= 20.4 mg-P/kg). Also, the P level of the same media at different depths was generally uniform after saturation (top and lower media mixture were 485 and 478 mg-P/kg-media; top and lower sand media were 133 and 148 mg-P/kg-media).

In conclusion, most of the P in the runoff was captured by the surface bioretention media first. After reaching a saturation point, P leached to the lower media and was retained. Overall, soil with higher P sorption capacity removed larger amounts of TP from runoff than the sand.

Media P Affiliations
The objectives of this task are to assess the P leaching potential and the P fertility of bioretention media after 16 repetitive 6-hr high-P loadings. The employed methods included environmental soil tests (WSP and CaCl₂ -P) and agronomic soil tests (Melich-I P and Mehlich-III P). WSP and CaCl₂-P were developed to simulate the ionic strength of the soil solution, predicting the potential risk of easily desorbable P leaching from the soil. Both Melich-I P and Mehlich-III P extractants are strong acid mixtures. The results are summarized in Table 1. Based on the results, the level of WSP increased for all testing media after 16 repetitions, and the distribution of WSP through the media depth is quite uniform (2.7 to 7.2 mg WSP/kg-media). For CaCl₂-P, only the top medium increased (from 0.2 to 2.6 mg CaCl₂-P/kg-media for the upper top-medium and from 0.2 to 0.8 mg CaCl₂-P/kg-media for the lower top-medium). Turning to Melich-I P, the level for both layers in top-medium did not have significant differences and the average increase is 30 mg Mehlich-I P/kg-media. Similarly, the average increase in Melich-I P was 11.5 mg P/kg-media for the middle-medium. A 24.3 mg of Mehlich-I P/kg-media increase was shown in the bottom layer. As expected, larger amounts of P were extracted by Melich-III extractant for all media. The average increase for each layer was 43.5 mg Mehlich-III P/kg-media for the top medium, 19.4 mg Mehlich-III P/kg-media for middle medium, and 66.9 mg Mehlich-III P/kg-media for the bottom layer. Overall, media with soil and mulch (top and bottom) contained larger amounts of these four types of P than the middle sand layer.

Several studies investigating the correlations between these P extractions and P leaching potential are summarized in Table 2. Comparing with Table 1, the WSP levels for all media were below the suggested value (8.6 mg WSP/kg soil). CaCl₂-P in the upper top-medium (2.6 mg CaCl₂-P/kg soil) was higher than the change point (1.59 mg CaCl₂-P/kg soil) and leaching probably caused the increase in the lower top-medium CaCl₂-P level (0.8 mg CaCl₂-P/kg soil). All media were below the change points of P leaching while using Mehlich-I (81 mg Mehlich-1 P/kg soil) and Mehlich-III (181 mg Mehlich-3 P/kg soil) extractants. In summary, the leaching potential of bioretention media after treating a total of 2 g P in 16 runoff applications is still under the risk point.

P is an essential plant nutrient, used for forming nucleic acids and ATP. P deficiency will result in stunted plant and growth delay. Understanding the relationship between P extracted by a given soil test and the expected plant yield can help to manage P in the field. Mehlich-I and Mehlich-III extractants are usually employed for P fertility tests. The optimum P range suggested by Sims et al. (2001) is 25 to 50 mg Mehlich-I P/kg soil and 50 to 100 mg Mehlich-III P/kg soil. Compared with the results in this study (Table 1), P fertility for all testing media was increased. Therefore, the contribution of retained P to bioretention media fertility was confirmed.

Tasks and activities for the next reporting period

Tasks for the next reporting period
The task for the next reporting period is to finish the final project report.

Work plan to accomplish tasks
The final report will be submitted by November 30, 2003.

Concerns or difficulties
At this point, we have no concerns or difficulties.

Expenditures
Expenditures are in the range expected for the work accomplished to date.

References
Maguire, R.O. and Sims, J.T. (2002) "Soil Testing to Predict Phosphorus Leaching," J. Environ. Qual., 31, 1601-1609.

Murphy, J. and Riley, J.P. (1962) "A Modified Single Solution Method for the Determination of Phosphate in Natural Waters," Anal. Chim. Acta., 27, 31-36.

Sims, J.T. and Heckendorn, S.E. (1991) Methods of Analysis of the University of Delaware Soil Testing Laboratory, Coop. Bull., 10, University of Delaware, Newark, DE.

Sims, J.T., Leytem, A.B., Gartley, K.L. (2001) Interpreting Soil Phosphorus Tests, Coop. Bull, NM-04, College of Agriculture and Natural Resources, University of Delaware, Newark, DE.