Subsurface infiltration and surface bioretention systems composed of engineered and/or native soils are preferred tools for stormwater management. However, the disturbance of native soils, especially during the process of adding amendments to improve infiltration rates and pollutant removal, may result in releases of nutrients in the early life of these systems. This project investigated the nutrient release from two soils, one disturbed and one undisturbed. The disturbed soil was collected intact, but had to be air-dried, and the columns repacked when soil shrinkage caused bypassing of water along the walls of the column. The undisturbed soil was collected and used intact, with no repacking. The disturbed soil showed elevated releases of nitrogen and phosphorus compared to the undisturbed soil for approximately 0.4 and 0.8 m of runoff loading, respectively. For the undisturbed soil, the nitrogen release was delayed, indicating that the soil disturbance accelerated the release of nitrogen into a very short time period. Leaving the soil undisturbed resulted in lower but still elevated effluent nitrogen concentrations over a longer period of time. For phosphorus, these results confirm prior research which demonstrated that the soil, if shown to be phosphorus-deficient during fertility testing, can remove phosphorus from runoff even when disturbed.
To decrease the volume of stormwater runoff reaching already-degraded urban streams, many localities in the US are either mandating or encouraging the use of green infrastructure. Infiltration is a primary component of green infrastructure/low-impact development because it restores some of the natural hydrologic function to urbanized areas by introducing water back to the groundwater, either through surface or subsurface devices. Infiltration systems also have the potential to remove some of the pollutants transported in urban runoff and reduce their discharge to surface receiving waters through the interaction of pollutants and the infiltration media. Many state guidance documents describe the ideal media characteristics for this pollutant removal.
One concern with the heavy reliance on infiltration systems for pollutant removal is the potential for groundwater contamination. Papers of Pitt et al. [
The focus of much of the research on groundwater contamination from stormwater infiltration has been on the fate of stormwater pollutants. Little attention has been paid to the components of the media mix itself. Guidance documents often specify that the native soil be incorporated into the media mixture. First, this assumes, or requires that testing demonstrate, that the native soil is not contaminated. Second, it assumes that disturbing the soil to incorporate more organic matter and/or sand for improved removal and hydraulic stability will not have negative impacts on the water passing through the filter.
Leaching of nutrients has been observed from newly constructed infiltration devices [
The soil selected for testing was a Wharton silt loam from central Pennsylvania. In the field, 21 ten-centimeter diameter columns of the soil were encased in 0.8 m length PVC pipe and removed intact from the sampling site. The collection location was a sloped field with a shallow soil that is less than 1 m to bedrock. Currently, the land is maintained as a lawn but there has been agricultural activity in the past and plowing may have occurred. The visible O horizon was 3 cm deep but was exaggerated to 7.5 cm in the O horizon columns to keep the soil intact; the final O horizon consisted of the visible O horizon and the transition to the A horizon. The A and B horizons were moderately rocky. Once the columns were returned to the laboratory, the soil profile was separated into layers by slicing off a portion of the top or bottom of the encased soil, depending on the horizon desired for testing. Five columns were used for each horizon group (O, A, AB, and entire profile) testing with one column used as the control or pretesting soil condition. Vegetation that was extracted with the columns was cut at the level of the soil surface and removed. The vegetation was not weeded because of the concern for disturbing the soil.
Within two days of returning the soils to the laboratory and separating the columns into the specified horizons, it was observed that the soil had shrunk away from the walls of the pipe in all columns. New samples were collected at the same location; however, even though they were covered to maintain moisture in the soil profile, shrinkage was observed. Therefore, a second local soil of similar quality for pH and organic content, as reported in the USDA/NRCS soil surveys, with similar geographic location and accessibility, was selected for comparison with the silt loam. The soil selected was a Leetonia loamy sand, again collected from central Pennsylvania (Table
Comparison of silt loam and loamy sand from USDA Soil Survey and analytical testing.
Silt loam | Loamy sand | |
---|---|---|
Soil pH | 4.0–5.0 | 3.6–5.0 |
Organic content | 1–4% | 1–5% |
Cation Exchange Capacity [CEC] (meq/100 g) | 3.8–8.0 | 0.6–2.0 |
Soil pH | O horizon: 4.5 | O horizon: 4.7 |
AB horizon: 5.7 | AB horizon: 4.7 | |
Organic content | O horizon: 5.5% | O horizon: 9.5% |
AB horizon: 1.8% | AB horizon: 1.4% | |
Cation Exchange Capacity [CEC] (meq/100 g) | O horizon: 19 | O horizon: 15 |
AB horizon: 12 | AB horizon: 11 | |
Total nitrogen (mg/kg) | O horizon: 2,900 | O horizon: 4,700 |
AB horizon: 1,000 | AB horizon: 700 | |
Total phosphorus (mg/kg) | O horizon: 35 | O horizon: 16 |
AB horizon: 5 | AB horizon: 2 |
As noted in Table
For the silt loam soil, the laboratory disturbance consisted of extracting the soil from the column, separating it into 7.5-cm layers, air drying, and repacking without compaction except from the weight of the soil above any layer. While this procedure is more rigorous in terms of not compacting the soil than the field construction of infiltration systems, it is similar in its intent.
The test water for this project was stormwater runoff collected from the Penn State Harrisburg campus. Approximately once a week, 600 mL (equivalent to 75 mm of runoff on the soil surface) was distributed into each column. Given that most infiltration systems are designed at a 5 : 1 or 10 : 1 loading ratio, this 75 mm of runoff on the soil surface is equivalent to 15 mm or 7.5 mm of runoff from a drainage area. These “events” are much smaller than a typical design runoff event; this small loading was selected in order to evaluate the change in nutrient release over much smaller time steps to determine the length of time (measured as a water loading) for which nutrient release could be expected. Infiltration through the columns was by gravity only; no artificial pressure was applied to either the top or bottom of the columns. Hydraulic head was maintained between 2.5 and 7.5 cm. Each soil type received a total of 40 simulated storm events over the course of one year.
Samples of the influent and effluent from each column were collected weekly and the effluent volumes recorded. Water quality tests included pH and conductivity, total hardness (calcium/magnesium) by titration, and turbidity, color, total nitrogen, total phosphorus (phosphate), potassium, and sulfur (sulfate). All samples were collected and analyzed according to approved US EPA protocols and/or
At the start and end of the project, plus four times throughout the project, a column of each representative test group (OAB, O, A, AB) was sacrificed for soil testing at the Penn State College of Agricultural Sciences Agricultural Analytical Services Lab. Each sacrificed column was subdivided into 7.5 cm segments and tested for soil pH, soluble salts, total carbon and total nitrogen through combustion, and phosphorus, potassium, magnesium, calcium, zinc, copper, and sulfur by Mehlich 3 extraction and ICP analysis.
The data below are presented as ratios of the effluent to influent concentration.
This paper focuses on the nutrient release from each of the two soil types and from the organic (O) and mineral (AB) horizons since nutrient release is the issue of concern for both surface and groundwater contamination. The full data set may be found in Treese [
Figure
Phosphorus export from (a) organic horizon and (b) mineral horizon.
Figure
Ratio of sample soil results for phosphorus for (a) disturbed O horizon, (b) undisturbed O horizon, (c) disturbed AB horizons, and (d) undisturbed AB horizons.
Generally, these results are in agreement with the literature on long-term phosphorus behavior in agricultural and forest soils, although no prior studies have investigated the short-term behavior of phosphorus at the resolution used in this study. The silt loam had higher initial phosphorus content than the loamy sand (twice as high), as would be expected since the loamy sand had more sand in the mixture. However, the loamy sand had a much higher initial organic content. For phosphorus, this higher initial concentration and the disturbance of the soil had a very limited impact (0.4 m of cumulative loading). This agrees with Boem et al. [
These results also illustrated the impact of the initial phosphorus content of soils on phosphorus release. Both soils had an initial release from the organic layer, and a removal from the passing water in the mineral layers. Increased retention of phosphorus by subsurface soils has been noted before in both forest soils [
The rapid decrease of the initially elevated effluent phosphorus concentrations in this study correlates well with the trends observed from other elements leached from the disturbed soil’s organic horizon plus the trends in the aggregate ionic measurement of conductivity (data not shown). For phosphorus, it appears that disturbance may have no impact on phosphorus release by itself, but instead the initial phosphorus release results from initial soil concentrations in excess of plant needs. Initial releases are substantially higher in both soils; however, the 0.4 m of runoff loading would be less runoff than would be expected during a single large storm.
Compared to the total phosphorus, the initial release of total nitrogen from both the organic and mineral horizons of the disturbed soil was very high (100 times the influent concentration) and lasted approximately twice as long (approximately 0.8 m of runoff loading), despite the lower initial concentration of nitrogen in the disturbed soil (Figure
Nitrogen export from (a) organic horizon and (b) mineral horizon.
Ratio of sample soil results for nitrogen for (a) disturbed O horizon, (b) undisturbed O horizon, (c) disturbed AB horizons, and (d) undisturbed AB horizons.
This higher effluent concentration from the disturbed soil columns occurs despite the initial O horizon nitrogen concentration and total organic content of the disturbed silt loam soil being approximately 60% of the concentration in the undisturbed loamy sand. A release of nitrogen from soils which were dried, sieved, and then repacked has been published and linked to bound nitrogen (12–27% contribution) in the upper 2 cm of tilled and no-till soils with greater release from the no-till soils of higher organic content [
The initial substantial leaching of some tested parameters by the silt loam soil columns, which had to be air-dried and repacked, may resemble what occurs after construction of infiltration units. An initial release of nutrients from infiltration system media has been observed before the establishment of vegetation [
Phosphorus leaching has been a problem in bioretention systems with underdrains and assumed to be due to disturbance [
This study, with its focus on the early life of soil media, indicates that there are potential concerns with nutrient releases during the early storm events. For infiltration in native soils, this study reinforces the need to evaluate whether soil disturbance is required or whether an area with good infiltration should be left undisturbed. When combined with the results of Clark and Pitt [