Water Quality: Turbidity

If you spend time on the waters of the Chesapeake Bay, you might notice that sometimes the water is crystal clear and sometimes it’s brown and clear as mud. When it comes to tracking water quality, measuring water clarity can help us start to get a picture of what is happening in the water.

One of the most common ways to do this is to record turbidity, which is a measure of how cloudy water is and therefore a proxy for how much “stuff” is in the water. The “stuff” can include phytoplankton, sediment, and other suspended particles. When sampling water quality in the field, we often use a very simple device for estimating turbidity called a secchi disk. A secchi disk is just a round plate attached to the end of a line. The plate has a specific black and white pattern on it and as it is lowered into the water, you look for the point at which you can no longer see the pattern. Once you’ve found this point, you mark where the surface of the water is on the line attached to the secchi disk and you measure the distance between that mark and the secchi disk to provide an inverse estimate of turbidity. The larger the number, the lower the turbidity of the water. This method is quick and cheap but is limited in that it really only provides an estimate of water clarity that can’t be compared to an actual turbidity measurement.

A secchi disk, a secchi disk in action, and a turbidity tube.

Other ways to measure turbidity include a turbidity tube, which is like a tall graduated cylinder with a secchi disk pattern at the bottom. You pour water into the tube until the pattern disappears and mark that height to get basically the same turbidity measurement as if you were using a secchi disk.

Higher tech devices, like a nephelometer, shine light through water and calculate turbidity based on how the light is scattered by the suspended particles. These devices measure turbidity in NTU (in the US) and FTU (Europe) and stand for Nephelometric Turbidity Units and Formazin Nephelometric Units, respectively.

While portable devices for measuring turbidity exist, they tend to be far more expensive, need to be calibrated to maintain accuracy, and can be sensitive to the environmental conditions inherent in working in the field. One of the main portable devices used is a YSI. While this is a brand name it’s also the term many people use to describe the handheld device and sonde that can measure a wide range of water quality parameters. However, these devices cost thousands of dollars for basic measurements (like pH and temperature) and the price goes up as you add measurement parameters (like dissolved oxygen, turbidity, chlorophyll a, etc – all of these parameters will be covered in this series).

Dr. Lycett using a YSI with the National Park Service's Assateague Water Quality Monitoring Program (circa 2012).

Turbidity as a component of water quality monitoring is important, but like salinity it is highly variable and can be dependent on a wide variety of other factors. For example, after it rains, turbidity tends to be higher as sediment and other particles are picked up by rain and run off into the Bay and other bodies of water. Thus, we know turbidity tends to be high for a period after rain events. Turbidity can also be high when there is a lot of phytoplankton present, whether due to natural periods of population growth or nutrient induced algal blooms. We tend to see more turbid water in the Chesapeake Bay in the spring and summer because phytoplankton are happily using increased sunlight to photosynthesize. Because they are the bottom of the aquatic food web, this means more food for other animals too, so normal phytoplankton growth is a good thing.

Unfortunately, when we have too many nutrients like fertilizers (nitrogen and phosphorus) in the Bay, we can see algal blooms where phytoplankton populations get out of control. However, when measuring turbidity alone, we can’t know if the suspended particles are plant-based or not – in order to know that we need to measure another parameter, chlorophyll a. We’ll be talking about this particular water quality measurement next month, in our March Heron Herald.

So, when phytoplankton populations are higher in the spring and summer, water clarity is decreased. In the fall, as sunlight is decreasing and land trees are changing leaf colors and losing their leaves entirely, we also see decreases in phytoplankton populations. This means the fall tends to be when we see some of the highest water clarity in the Chesapeake Bay. We’ll also see higher turbidity when we have frequent rain and lower turbidity during dry spells. These are all normal changes in turbidity.

For example, you might have seen posts from our local Riverkeepers about the extreme water clarity this past fall. Being able to see 9+ feet in the Chesapeake Bay is rare but is most likely to happen in the fall.

Unfortunately, humans can also increase the turbidity in a number of ways that lead to poor water quality. When rain falls on land, it moves downhill, carrying with it loose particles. Construction sites and other areas when surface plants have been removed tend to just have loose dirt instead, which means the rain can carry away a lot of sediment. This is why it’s important to have sediment barriers around construction sites, to help reduce the amount of sediment that’s ending up in the run off.

We see the same thing happen in parking lots and other places with a lot of impervious surfaces, areas where the surface does not allow water to drain into the soil. That water runs off, taking whatever debris (trash, particulates from vehicle exhaust, dirt, etc.) with it into the nearest body of water. In these cases, rather than use a sediment barrier like at a construction site, we can use built natural structures like bioswales, rain gardens, and stormwater management ponds to help collect the water and let it percolate down into the soil instead of entering the Chesapeake Bay. This also gives plants in those areas time to absorb excess nutrients that might be in that run off and allows sediment to settle out as well.

The Chesapeake Bay Watershed Agreement does not have specific goals for turbidity alone, but rather calls for improving water quality to support healthy fisheries and clean water. However, they do have specific goals for reducing some of the types of suspended particles that affect turbidity, like sediment and nutrients. We have what are known as Total Maximum Daily Loads (TMDLs) of different materials that we are allowed to release into the Chesapeake Bay watershed and still maintain good water quality. Across the entire Chesapeake Bay watershed, the goal yearly limit for nitrogen is 189.5 million pounds. For phosphorus, it’s 12.5 million pounds and for sediment it’s 6.45 billion pounds. These numbers are based on a 25% reduction in nitrogen from 2009 levels, 24% reduction in phosphorus, and a 20% reduction in sediment.

These goals and the steps to achieve them are outlined by Watershed Implementation Plans. These are documents created by each state of the 6 states in the Chesapeake Bay watershed and the District of Columbia. Each WIP is expected to be refined and adjusted at regular intervals so that the process for achieving these goals changes in response to changes in water quality and best practices for improving water quality. To simplify, each state has a plan for reducing their TMDLs and these plans change with the latest data. The Environmental Protection Agency is responsible for holding states accountable if these plans are not followed. So what have been the results of these plans? To answer this, we turn to Chesapeake Progress, which is a tracking tool put together by the Chesapeake Bay Program.

To see the full document, click the image above.

The expectation was that from 2010 to 2025, we would reduce the amount of nitrogen, phosphorus, and sediment going into the bay and reach these yearly goals by the 2025 deadline. While we’ve made some progress, especially in reducing the amount of nitrogen and phosphorus entering the Bay through wastewater treatment facilities, we are still short of these targets. As of 2022, we’ve achieved just over half of the target reduction in nitrogen, 60% of the target reduction in phosphorus, and about 5% of the target reduction in sediment.

There’s a lot of technical information about all this but some of the big takeaways are that we’re seeing an increasing amount of nitrogen entering the Bay through septic systems and developed land, even as we reduced how much is entering through agricultural land and wastewater. For phosphorus, we’ve decreased the amount entering the Bay each year from agricultural land and wastewater but we’re seeing an increase in the amount coming off developed land. For sediment, the biggest decreases have been from agricultural land but we’re seeing increases coming from developed land.

So, what can we do? Basically, wastewater is still a huge source of nitrogen and phosphorus loading in the Bay, even though we’ve made some huge reductions in that area. We need technology that continues to advance those reduction efforts. We’ll also need to continue the decreases in nitrogen, phosphorus, and sediment from agricultural lands. Even though we’ve seen reductions across all three, agricultural land is still the biggest source of nitrogen and phosphorus and the second biggest source of sediment coming into the Bay. This is why lots of folks are working towards continuing to implement what are called BMPs – Best Management Practices – on agricultural lands. This includes things like winter cover crops to hold soil in place during wet winter months, buffer zones along the edges of farmland to help slow and trap water and everything it carries with it, and soil testing requirements before fertilizers can be added.

We also need to improve nutrient and sediment loading from developed lands as the amount of developed land in the Chesapeake Bay watershed continues to increase. Using surfaces that allow water to percolate into soils, instead of impervious surfaces in developed areas can help. This includes things like reducing the amount of pavement and other hard surfaces by utilizing building features such as green roofs, rain barrels and other rainwater collection systems, permeable driveways, and permeable pavement.

All of these BMPs and more can help us reduce the amount of “stuff” ending up in the Chesapeake Bay and help us achieve healthy water clarity throughout the year. What practices do you utilize that helps reduce runoff in any form, whether at work, home, or school?

A bioswale at Salisbury University

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