Turbidity Trouble

I was looking at the news the other night and there was footage of yet another mudslide and another flooding river. There's been a lot of rain in the area this month; more than usual. Watching footage of the muddy brown water I couldn't help but think of the turbidity issues we're struggling with on a couple projects in Issaquah.

Turbidity is the measure of particulates suspended in water. It's measured using a turbidimeter (say that three times fast). A turbidimeter works by sending light through a sample contained in a glass vial and measuring how much actually gets through and how much is deflected by suspended particles. The turbidity of the sample is then calculated in NTU's (Nephelometric Turbidity Units) based on the amount of light deflected. The more suspended particles there are in the sample, the higher the turbidity.

For kicks, here are trubidity measurements for a couple of things around the office
Tap water = 0.5 NTU
Filtered water from the fridge = 0.15 NTU
Regular coffee = 34 NTU
Decaf = 25 NTU

Even though the coffee is dark, it has a relatively low turbidity reading because the coloring is caused by something dissolved (like a dye) and not suspended in the water. Now if your coffee filter is old, or if you actually like your coffee with the grounds you can expect a much higher reading.

The City of Issaquah has a turbidity limit for construction sites of 100 NTU in the wet weather season, which out here runs from October 1 through April 31. This is no different from the County turbidity limit which also calls for a maximum of 100 NTU. But in Issaquah they up the ante: the penalty for exceeding that limit is a red-tag and a $5k fine. All work on site comes to a screeching halt until the NTU's clear, the city is paid off and a new work plan is submitted along with a formal apology for being a bad contractor.

It's not cheap to keep your NTU's below 100 when you're working outside and it's raining. Our sites are fairly large and they're in all stages of construction from building foundations to final finish and landscaping. In order to meet the standard we've got at each site: a full time employee to sweep all roads clear of dirt and take twice daily turbidity readings keeping logs of findings, large holding tanks and sand filters to contain runoff and filter out sediment, crushed concrete and straw bales over bare flat ground and plastic on all unplanted slopes to keep the soil from washing away.

Between the cost of the tanks, sand filters, new employees, ground protection, not to mention the fines... but that's the price we pay for a healthy ecosystem right?

But then I was watching the news with all that muddy water on the screen, and rusted out cars under floodwaters in backyards, and a couch floating by... and I just wondered.

BuilderGrrl & the Flying Fish