Coastal Chemistry Corner

All Roads Lead to Reef

After heavy rainfall in North Kohala caused flash flooding, rockslides, and road closures in early May, the waters in and around Kawaihae Harbor turned brown for several days. A large runoff plume extending up the coast could even be seen from space. Such large storms are generally short-lived and infrequent in West Hawaiʻi, but they produce visible reminders of the ever-present connection between what we do on land and the condition of our coastal waters.

Twenty years of monitoring data from West Hawaiʻi shows that the amount of developed land upstream of a reef is the second leading cause of coral’s inability to withstand heat waves (after wastewater). But how do urban areas actually impact coral? What is it about roads and parking lots that results in poor water quality?

While most types of paving materials are not themselves a large source of environmental pollution, stormwater runoff from paved areas is more toxic and destructive compared to runoff from natural areas. Over time, vehicle traffic results in build-up of heavy metals and combustion byproducts from car exhaust, wearing of brake pads and tires, and leaking fluids. That contaminant-laden road debris eventually gets washed downstream to the coast in stormwater runoff. Harmful effects to coral health after exposure to heavy metals and even trace amounts of hydrocarbons are well-documented, ranging from chronic sub-lethal effects like stunted growth to acute toxicity and death.

In addition to funneling loads of toxic contaminants downstream, pavement also transforms the landscape in a way that increases the volume and erosive power of stormwater runoff. Compared to areas like porous volcanic rock where rainwater naturally soaks into the ground, impervious pavement doesn’t let water infiltrate so even small storms generate surface runoff. This re-plumbing of the landscape means more rainwater gets washed quickly over land and carries eroded sediment downstream, resulting in murky coastal waters rather than slow percolation underground with natural filtering by microbes and bacteria. Large volumes of freshwater can also reduce the salinity of receiving waters to levels that prevent fertilization during coral spawning.

Fortunately, there are many strategies for mitigating impacts of stormwater runoff from developed areas. These “low impact development” practices are typically decentralized, small-scale infrastructure designed to slow down, retain, and filter rainwater where it falls and minimize the total impervious area connected to stormwater flowpaths, such as permeable pavement, vegetated buffers, and rain gardens. They can be incorporated into the designs for new construction or applied as retrofits to existing development. Picking up pet waste and litter, and only using chemical fertilizers and pesticides on an as-needed basis will help reduce the overall load of contaminants that can get washed downstream. Runoff from areas like gas stations, repair shops, and industrial areas will have more concentrated pollutant loads that should be pre-treated and filtered before being directed to stormwater drainage. And to minimize the impacts of road travel, keep up with regular vehicle maintenance to make sure exhaust systems are functioning properly and to avoid leaking motor oil and other fluids.