At about 2.5% of the total water volume on the planet, we’ve always had roughly the same amount of freshwater. Unfortunately, it seems that, at the local level, the amount of fresh water made available through precipitation is increasingly erratic, with the last year featuring historic floods in the eastern US and historic drought in the west. In my adopted home state of California, 2013 was officially the driest year on record and snowpack, groundwater and reservoir levels throughout the state are critically low. Although we’ve undertaken extensive engineering feats in the form of reservoirs, diversions and water supply pipelines, local water management decisions provide our greatest leverage on local water supply.
Stormwater infiltration is defined as the process by which water enters the soil and recharges streams, lakes, rivers, and underground aquifers. Stormwater infiltration is a fundamental component of the water cycle and is quickly becoming the centerpiece of stormwater management strategies across the United States. Stormwater infiltration is an effective means of managing runoff because it allows practitioners to address both water quality and water quantity concerns.
Bioretention and green roofs have become the centerpieces of Low Impact Development (LID) initiatives throughout North America. The well-publicized benefits of these two types of stormwater management practices focus on runoff reduction, stormwater quality treatment, and landscape aesthetics. Promotional literature provided by various regulatory agencies and environmental organizations typically highlights the processes and mechanisms within bioretention and green roofs that provide desirable outcomes. Normally, one finds mention of evapotranspiration, filtering, and adsorption characteristics of the vegetation and soil mix as beneficial mechanisms for the purposes of runoff reduction, particulate and hydrocarbon removal, and dissolved pollutant capture, respectively.
System hydraulics refers to how water flows through a stormwater filtration system. The following three steps should be part of the hydraulics evaluation process.
During the past decade, a number of different media have been used for stormwater filtration and more recently for bioretention. Media such as sand, peat, and compost have been used successfully. Uses of perlite, zeolite, carbon, and other "exotic" media have expanded the choices for targeting specific pollutants. Media are now being used to target TSS, Petroleum Hydrocarbons, Dissolved Phosphorus, dissolved metals and even bacteria. More recent research in biofiltration adds elements of biological uptake of nutrients and metals by plants, conversion of Nitrogen into ammonia in anaerobic zones.
Since the Environmental Protection Agency, and in turn state and local regulators, have gone all in on green infrastructure (GI) and low impact development (LID) concepts a seemingly regrettable consequence has emerged. Topics specific to the broader adoption and implementation of GI have monopolized our collective dialog on stormwater management of late. The predominantly positive press and barrage of GI heavy conference agendas seemingly suggest that if we apply GI far and wide then water quality impairments caused by urban runoff will soon be a thing of the past. GI provides stormwater practitioners with invaluable tools to aid in the stormwater fight, but many site specific challenges can’t be surmounted with GI solutions alone. To that end, should we be concerned that overemphasis on GI is discouraging innovation?
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