It has been noted that humans are important geological agents. Currently, half of the worlds population lives in cities which require vast amounts of crushed rock, iron, copper, and various other inorganic substances. My favorite undergrad professor would always say "what can't be grown must be mined." Humans have leveled mountains for ore, created land by filling marshes, and changed the movement and quality of water on and under the ground surface.
Urban watersheds are a hot topic of research. As we build cities, we cover streams and ponds with impervious surfaces. Stream networks that once had several tens of tributaries have been reduced to a few that can be counted on your fingers because people don't want a stream running through their business or house. This puts greater strain on the remaining tributaries which can lead to flooding and faster rates of erosion. To get around this in San Jose, they have concrete-lined many if not all of the streams in the city. The concrete reduces erosion of the banks, and it creates laminar flow which requires more energy to flood. While the people and business are mostly safe, the streams are ugly and they reduce the recharge of the groundwater which is an important function of streams.
As cities grow, we install more underground utilities - conduits for gas and electric lines, water, internet, and subways. The outside of these tunnels/channels are often filled with gravel. Gravel drains well and is usually abundant. Scientists have dubbed this 'urban karst.' Karst is the geologic phenomena generally responsible for forming caves - such as Mammoth Cave in Kentucky. One scientists estimated that the percent of channels in Mammoth Cave is approximately equal to the percent of channels under Quebec, Canada. Considering the shear size of Mammoth Cave, that is astounding. So, cities have taken away surface water and exchanged it for subsurface water. As a rule of thumb, scientists estimate water utilities leak at least 6%. Adding this to our urban karst plus all of the water we use for our lawns, washing our cars, etc - Groundwater recharge under cities is actually increasing. This can be a good or a bad thing - depending on the quality (i.e. cleanliness) of the recharged water.
Rainwater runoff is a hot topic in the Bay Area because we are trying to protect the bay as a natural area. It is completely surrounded by cities. Imagine your daily life - every time you drive, incomplete combustion from your engine releases organic particulates that settle onto the roads. Furthermore, your tires eventually wear down. They don't disappear by magic - they leave bits of tire everywhere you've been. When you brake, bits of your brake pads spew onto the roads. These little bits add up. When it rains, water picks up these particulates and carries them 'away'. Some runoff in the Bay Area is treated because it is mandated. In other cities, the runoff washes to the side of the road where the particulates percolate down into the soil, streams, and groundwater. You might think the journey is over - but alas it has only just begun. One group in Illinois took on an interesting project. When it snows in the winter, the city sprays salt on the roads to reduce the ice. This group monitored salt for four years in a nearby marsh. They compared the marsh water to our drinking water standards, and they found that the salt lasts in the marsh for at least a year in quantities far greater than you and I would drink. And as last years salt is dwindling, the winter snows return bringing more applied salt.
But all is not lost. Scientists and city planners are beginning to think about groundwater flow and it's relationship to natural areas. A park in Alberta, Canada that is surrounded by a city modeled their subsurface groundwater flow. The modeling software, a standard for hydrology, is capable of tracking the flow of the water before it gets to the park. The park was able to determine the source of the groundwater flowing under their park. Amazingly enough, even though the source of the water was not in the park, city regulators are beginning to discuss ways of protecting the integrity of the water before it reaches the park!
Another group is studying permeable pavements. These are quite useful for reducing puddles on the road, and increasing visibility during rain storms. They are often a 2 inch layer of loosely packed crushed rock and some sort of binding agent to keep the rock together on top of traditional asphalt. As rain percolates through the 2 inch layer, particulates from our cars are trapped, which greatly improves the quality/cleanliness of the road runoff. It seems like a miracle fix for non-icy climates, however it has to be replaced every 10 years or so, but it is often recycled and used again in another permeable surface. There are other kinks to work out, but it seems like people are working to keep our urban water clean.
I learned quite a lot at the conference, and I really enjoyed the city of Denver. The week before we were there, it snowed in the Rocky Mountains. It was a beautiful sight from the city. Just think, in a few months time, that snow will be someone's drinking water. As snow melt, it has a long way to travel before it gets to someone's glass.
Thank you Sarah for sharing all you learned at the con ference. Every city has it's own unique geologic structure in its local watershed, but stormwater quality is uniformly not good for health ecosystems anywhere.
ReplyDeleteBack home in San Jose, about half of our urbanized watershed is underlain with a thick deposit of clay, which acts as an aquitard. Consequently, most of the streams in the lowlands, generally a mile or so north of Hwy 85, act to drain shallow ground water from below the hardscape. Some of that is irrigation water, pipeline leakage, and some of rainfall which still gets into the ground. This happens in the Coyote Creek, which is still pretty natural, and even in and under concrete lined channels, which I agree are ugly. Upstream of that aquitard, however, recharge does happen to a greater degree and that is the conduit for stormwater pollution yo impact of groundwater quality, Pollution prevention should be focused here to protect the basin.