Evaluation of Biochar for use as a Roadside Soil Amendment for Stormwater Runoff Management

Roadway stormwater runoff is a significant contributor of nutrients, such as nitrogen and phosphorus, to waterways. Elevated levels of nutrients threaten the health of water bodies by accelerating eutrophication. To manage stormwater runoff, soil amendments can be added to roadway soils. Compost is a common soil amendment as it promotes infiltration and re-vegetation. However, compost may be a source of leachable nutrients. Biochar, a charcoal-like material made from pyrolyzed organic matter, could be an alternative as it also encourages infiltration and re-vegetation in addition to impacting nitrogen and phosphorus transport. While this is promising, there is still much to learn about how aspects of this system such as stormwater composition, climate change, and hydraulic soil group could affect nutrient transport in amended soils.

Nitrogen leaching from biochar or compost-amended roadside soils could be influenced by nitrogen and salt concentrations in stormwater runoff. To understand how stormwater composition may influence nitrogen leaching, we conducted a batch leaching test. This test quantified nitrogen leaching from pine biochar, buckthorn biochar, and compost in DI water and three artificial stormwater mixtures. Artificial stormwater mixtures varied in salt and nitrogen concentration. In general, total dissolved nitrogen (TDN) leaching varied by material and synthetic stormwater. For pine biochar and compost, TDN leaching decreased with stormwater ionic strength and increased with stormwater nitrogen concentration. Compost leached more total dissolved nitrogen than biochar regardless of leachate. Typically, buckthorn biochar leached less TDN than the pine biochar. This research highlighted that DI water results in conservative leaching estimates.

Climate change driven increases to storm volume and antecedent dry period could aggravate nitrogen leaching from roadside soil amendments, like compost and biochar. Despite this, neither amendment has been evaluated under these climate change scenarios. We compared effluent nitrogen and phosphate concentrations and masses following a series of simulated large volume storm events and a series of intermittent storm events with increasing antecedent dry periods. First, columns of roadside soil and biochar (30% by volume & 4% by weight) or compost (30% by volume) were irrigated to simulate either a 1 in storm event or 1 yr-6 hr storm events for Virginia. Then, columns underwent a series of 1 yr-6 hr storm events with increasing ADPs. Outflow was analyzed for TDN, nitrate, and phosphate. Columns were routinely weighed to gravimetrically track soil moisture. Results indicated that compost-amended soils consistently had the highest effluent TDN concentrations and masses in large storm events and increased ADPs. 30% biochar by volume consistently had the lowest effluent TDN concentrations in both storm series. Notably, in the 1yr-6 hr storm event, 30% biochar had the lowest effluent TDN mass. All amendments reduced phosphate transport in larger storm volumes but there were varying results under increased ADPs. All amendments improved soil moisture with 30% biochar by volume having the greatest effect on soil moisture.

Roadside soil amendments such as biochar and compost only alter the hydrologic characteristics of the top layer of soils. Infiltration rates of the underlying soil could still impact nitrogen transformations in this system. Previous studies have focused on nitrogen transport in only the amended layer of agricultural soils and not how hydrologic characteristics of the subsoils influence nitrogen transport. This chapter explores the relationship between hydrologic soil group and nitrogen transport in amended roadside soils. Columns of roadside soil and biochar (30% by volume & 4% by weight) or compost (30% by volume) underwent an artificial storm event in which synthetic stormwater was held in the columns for different durations that correspond to the four hydrologic soil groups. Generally, there was not a clear trend between hydrologic soil group, material, and nitrogen leaching. 30% biochar by volume and roadway soils had similar effluent TDN concentrations across hydrologic soil groups. As biochar performed similarly to roadway soils in all four hydrologic soil groups, it may be suitable for use as a roadside soil amendment. However, these results were limited by the number of replicates and this study would benefit from additional tests.

Collectively, these results suggest that biochar may be suitable for use as roadside soil amendment. Future work consists of evaluating the role of vegetation in this system and exploring the transport of other environmental contaminants. This dissertation addressed important questions regarding the influence of biochar and compost on nutrient transport in roadside soils. By investigating questions related to the influence of stormwater composition, climate change, and hydrologic soil group on nutrient transport in amended roadside soils, the findings of this dissertation can be applied to improve implementation of these materials.