Constructed Climates

Figure 1.12: Sedimentation rates in Pamlico Sound on the the North Carolina coast increased greatly over the last half century (after Cooper et al. 2004). Durham County’s Eno River and Falls Lake reservoir, as part of the Neuse Watershed, ultimately empties into Pamlico Sound. Agriculture, mining, military activities, and urbanization all play a role in this sedimentation, and reservoirs attempt to reduce it. We also drink these waters. Year 2000 per capita public-supplied freshwater use is for 2,384 counties in the United States plotted against its population size (data from water.usgs.gov). Squares indicate the 100 counties in North Carolina. In terms of filtration, things like infiltration trenches (grassy strips surrounding parking lots) and bioretention filters (water-holding gravel filled pits covered with grass and trees) work great (after Weiss et al. 2007).

Stormwater carries pollutants and sediments downstream, and when the water flow decreases, the heavier-than-water sediments fall out, just like the sand being carried in high winds drops to the ground when the air calms. Half of Durham sits in the Neuse River basin, meaning half of the city’s stormwater empties out into Pamlico Sound at the North Carolina coast.[53] Sediment cores taken in 1997 (Figure 1.12) show that as much as 2 cm of sediment, nearly an inch, drops into the sound each year! That’s at least 10 times more than three centuries ago, with most of that increase taking place in the last half century. Pity the critters living at the sound’s bottom.[54]

Across the United States in 2000, the average county had just under 71,000 people with publicly supplied water, and the average such-supplied person in the average county uses about 213 gallons of publicly supplied water per day.[55] Durham County had 167,000 people using public water, with 183 gallons per person per day. At a density of one person per 0.83 acre, 183 gallons spreads out to a 0.2 mm layer on each person’s area each and every day. Over one year that equals 7.3 cm — 3 inches — of rain. Durham gets about 48 inches of rain per year, so its citizens take just over 6% of the total rainfall for their personal use.[56] To meet this demand, we fill reservoirs from rain falling on impervious parking lots and tire-abrading freeways. Alternatively, we could preserve rural open space that serves as the watersheds for our drinking water supplies, letting forests and stream buffers filter rainwater. Since we lose about 60% of rainfall to evapotranspiration (Figure 2.14), at least 12% of the county’s forested area must be preserved as watersheds. Durham County uses both approaches, preserving rural open space as well as cleaning parking lot runoff.[57]

Constructed wetlands that include trees can serve many purposes: purify drinking water by filtering stormwater, cool the city, support wildlife, as well as provide recreation and environmental education.

The bottom graph of Figure 1.12 compares many types of stormwater treatment approaches.[58] Many competing aspects favor one type of stormwater system or the other. Construction cost data show that bigger is cheaper for all stormwater systems, and annual maintenance costs run about 3 to 5% of construction costs for large systems. One notable exception is that dry extended basins require very low maintenance costs whatever the size. Constructed wetlands are an overall good solution, nicely solving the stormwater filtration problem while promoting wildlife. An integrated approach to a city’s stormwater drainage system would be nice, a task complicated by competing goals of different city and county departments. Indeed, if the toxicity levels weren’t too high, children could also chase frogs and salamanders in these places.

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[53]Keep in mind the differences between the many types of “used” water: stormwater, wastewater or sewage, groundwater, drinking water, and reservoirs. Rain falls on impervious surfaces draining to streams, and it becomes stormwater that refills reservoirs. Rain falling on permeable surfaces soak in and can recharge groundwater sources, though some rain runs off permeable surfaces, too, and can make its way to streams. Reservoirs recharge from water flowing through streams, which can mean stormwater flushed into urban streams. Wastewater (or sewage) is the stuff flushed down toilets, showers, and sinks, partly cleaned in treatment plants before entering streams. Yet another term is graywater, water coming from sinks, washing machines, and, in some jurisdictions, also showers and bathtubs. Groundwater comes from wells drilled into aquifers, which get recharged by rainwater filtering through the ground throughout watersheds. Drinking water can come from both reservoirs and groundwater sources.

[54]Cooper et al. (2004) present the work on sedimentation rates in Pamlico Sound. They also show changes in the ecological community through several centuries as the stormwater sedimentation changed.

[55]Water use for each county was found at water.usgs.gov/watuse/data/2000/index.html. Another interesting calculation is how much of our total rainfall we humans use, specifically Americans. NOAA reports that the total precipitation for 48 states averaged about 29 inches per year over the last 100 years (www.ncdc.noaa.gov/img/climate/research/2006/ann/us-summary.html). The United States Geological Survey reports that water use across the United States equals about 408 billion gallons per day for all uses from drinking water to power plant cooling (pubs.usgs.gov/circ/2004/circ1268). Sometimes these uses reuse water already used for another purpose, but this latter number means 1.5 x 1014 gallons per year, or 5.5 billion acre–inches. The 48 states have about 1.9 billion acres, meaning we use about 2.9 inches of water covering that entire area each year. That’s our water footprint. In other words, humans use 10% of the U.S. rainfall, a bit more than Durhamites use.

[56]This water-use calculation can go in many directions. Here’s another. Divide the 408 billion gallons per day number by the U.S. population, 300 million, to get 1,360 gallons per day per person. That’s six times more than I calculated in the text for Durham citizens, leading to about 18 inches of rain per year, or nearly 40% of Durham County’s rainfall. That’s a seriously big fraction. The difference represents personal versus nonpersonal use.

[57]Yet another interesting feature of Durham County: Its urban streams help fill Falls Lake, a reservoir serving drinking water to Wake County, home to Raleigh, North Carolina. Preservation of open space and retrofitting impervious surfaces for better stormwater infiltration in Durham County becomes an important issue for another county’s citizens. Who pays? Although the Army Corps of Engineers acquired much of the land immediately surrounding the reservoir, these results imply that further development of upland areas reduces water quality.

[58]Weiss et al. (2007) compare many types of stormwater treatment approaches, concluding that constructed wetlands are the best solution. These types include “dry extended detention basins” (a barren hole) that hold water for less than two days; “wet retention basins” (a pond) that might hold water almost continuously; “constructed wetlands” (pond and meadows) that hold water for an intermediate time; and “infiltration trenches” (parking lot strips), which are pretty much anything that filters stormwater and prevents it from entering streams. The tree-planting locations in the Food Lion parking lot (see Fig. 2.12) could act as an infiltration trench if only the surrounding curbs were removed to allow runoff to reach those permeable areas.