SUVA and Total DOC

Dissolved organic matter is assessed in several ways.  Total dissolved organic carbon (DOC) is determined by high temperature catalytic oxidation.  At high temperatures, organic carbon is oxidized to carbon dioxide.  The instrument then measures the total carbon dioxide produced from a known sample volume.  Total DOC measurements are reported as concentration.  Total organic carbon fluxes can be determined from discharge data.

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Ion Chromatography

In order to describe the dissolved matter in agricultural runoff, we analyze the filtrate in several ways.  The first of these is ion chromatography for anions.  An ion chromatograph (IC) functions by transporting sample in an eluent through a column.  The column is packed with material that attracts the anions in the solution.  Since each ion has a different level of affinity for the column material, each anion species exits the column at a different time after injection.  The instrument measures the conductivity of the liquid exiting the column over time.  On a graph, this translates into peaks which correspond to different ions.  Peak areas are calibrated using known standards at a range of concentrations.  The anions calibrated for are: fluoride, chloride, nitrite, bicarbonate, sulfate, nitrate, and phosphate.  The standard mixed from ultra pure water and the salts of the relevant anions.

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Filtering Procedure

Now that we have samples, the next step is to filter them.  Because organic matter can break down rapidly, runoff samples need to be processed as quickly as possible.  We filter about 150 mL of each sample through 0.7 µm glass fiber filters.  The filters are pre-combusted in a furnace to destroy any organic contaminants.  The filtration apparatus is thoroughly rinsed between each sample.  Ultra pure (Milli-Q) water is passed through the filter paper before any sample.  The filtration is expedited by a vacuum pump (Büchner-style).

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A Good Storm

All summer we have been diligently watching the radar, hoping for a storm big enough to generate runoff and give us samples to analyze. We collect samples from a channel adjacent to an agricultural field. This channel only has water in it following sizable storm events. There have been several disappointing cases already. A storm passes over our field site, we head out, and the ground is barely damp. Finally, a large storm showed up on the radar. It rained here in Williamsburg as well – you may remember it. In the middle of the mess, Dylan (one of my fellow student researchers) and I loaded up the van and headed out the Charles City County at midnight in the pouring rain. On reaching our field, we took our equipment in a wagon and trekked around the field. We were aided by the light of headlamps and the occasional flash of (fortunately distant) lightening. We reached our collection site to find only significant ponding. The rain was letting up, and the radar seemed to indicate that there was not much left to the storm. We arrived back on campus around 2:30 a.m. We were tired, wet and a little dejected. Another disappointment – or so it seemed.
A few days later we went out to our field site for a routine check of sampling equipment. Much to our surprise we found that samples had been collected! It turns out that despite the lack of indications there was a small amount of flow in our channel that began a little after 2 a.m. While it certainly would have been better to catch these samples fresh, we are very excited to finally have some good samples to analyze. The samples look a little like iced tea – which is great news for DOC. I’ll be spending the week analyzing them!

Analysis of Dissolved Organic Matter in Agricultural Runoff

Dissolved organic matter (DOM) is the variety of molecules composed primarily of carbon, hydrogen in surface waters and groundwater that are typically leached from plants and soils. This mobile group of molecules plays a key role in biogeochemical processes, ecological processes, and the global carbon cycle. The composition of this organic matter can vary greatly depending on its original source and the degree to which it is broken down by microbial activity.

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