Cheney Reservoir, Kansas: 20 Years of USGS Water-Quality Studies

By Jennifer L. Graham, Guy M. Foster and Ariele R. Kramer, U.S. Geological Survey, with the City of Wichita. Fact Sheet 2017–3019, March 2017.

The North Fork Ninnescah River, which feeds the reservoir, in June and November. USGS photographs.

Why the reservoir matters

Cheney Reservoir, in south-central Kansas, is one of Wichita's main drinking-water sources and a regional recreation spot; with a growing city, it will stay that way.

• 1990 and 1991: cyanobacterial blooms caused severe taste-and-odor problems.
• 1992: the Cheney Reservoir Task Force formed. It named nutrients and sediment the main pollutants — for their effect on water quality and quantity and their link to blooms — and set water-quality goals for streams (1994).
• Since 1996: the USGS, with the City of Wichita, has studied the watershed.

Figure 1: USGS study sites, 1996–2016 — short-term (1996–2001) and long-term, continuous monitoring. USGS.

The watershed

No clear pattern in nutrient or sediment levels showed up from 1996 through 2013, mostly because of year-to-year swings in rain and flow. Judging the long-term effect of investment in best management practices (BMPs) will take more rigorous analysis.

Phosphorus

• Farming has raised phosphorus in the soil to about three times natural levels; about 65 percent of the phosphorus reaching the reservoir comes from agriculture.
• Sediment cores show phosphorus rising through the reservoir's life (1965–98), likely from more fertilizer and livestock.
• Levels run higher in the east of the watershed, lower in the west.
• 1997–2000: five subwatersheds ran two to five times the task force's long-term goal of 0.1 milligrams per liter, and natural levels in three of the five met or exceeded it.
• 1997–2012: the goal was exceeded about 60 percent of the time.

So: the goals may be out of reach without a lot more BMPs in the east.

Figure 2: measured (1997–2000) and estimated natural phosphorus at five sites, against the 0.1 mg/L goal (dashed). USGS, after Pope and others (2002).

Sediment

• As of 2001, sediment had taken only 4 percent of the reservoir's original storage; from 1965 through 1998, just 27 percent of the inactive conservation pool had filled — less than the 34 percent designed for.
• It comes in bursts: forty-one percent of the sediment from 1966 through 2013 arrived on just 8 days.
• 1979 alone brought 20 percent of 48 years' sediment — ninety-two percent of it in one day, during an approximately 100-year flood.

So: sediment plans must deal with big, rare floods.

Figure 3: annual suspended-sediment load, 1966–2013. USGS, after Stone and others (2015).

The reservoir

How often found, 2001–2016:

Microcystin and geosmin passed advisory levels more often than cyanobacteria counts did, so cyanobacteria alone are a poor warning sign. Problems here are episodic, not chronic.

Figure 4: cyanobacteria, microcystin and geosmin by month, 2001–2016 — medians (bars) and maximums (lines), against KDHE and EPA guidance and the human detection threshold. USGS.

The seasons

• Cyanobacteria: peak in late summer or early fall, with smaller winter peaks — though amounts vary by orders of magnitude between years.
• Microcystin: first appears in June or July, peaks in summer, then falls.
• Geosmin: less regular — usually a small winter peak, sometimes a bigger summer one. Summer peaks were rare until 2013, when an August inflow set off the highest late-summer and fall levels of 2001–2016; they have come every late summer and fall since. Changing conditions change the seasons.

Who makes them

Genetic tests point to Microcystis as the likely microcystin maker and Anabaena as the geosmin maker. The MIB source is unknown — perhaps cyanobacteria living on the bottom rather than floating, which haven't been studied here. Knowing the culprits lets scientists focus, and their presence may warn of an event.

Light, nutrients and sediment

The algae track nutrients (which feed them) up, and sediment (which blocks light) down. Growth here is likely limited by light, not nutrients — so cutting sediment without cutting nutrients could mean more blooms. In the end, biology, chemistry and hydrology all interact.

Real-time warnings

Water quality has been measured hourly since 2001 as an early warning for the treatment plant. Models estimate the chance of microcystin and geosmin, posted hourly on the USGS National Real-Time Water Quality site. The microcystin model has held up; the geosmin model has had to change with conditions — models need regular re-checking.

Cheney Reservoir in January and August. USGS photographs.

What's been done, and what's next

• 1994–2011: about 1,500 conservation contracts in the watershed, and about 20,475 acres more in the Conservation Reserve Program.
• Twenty years of data now let scientists document how that long-term strategy changes inflows and the reservoir.
• As of 2017, the studies were to run through 2018: continued monitoring at the reservoir's inflow and outflow, tracking change against BMPs, and linking inflow to reservoir processes to learn what makes a bloom year.

Sources

Based on Twenty Years of Water-Quality Studies in the Cheney Reservoir Watershed, Kansas, 1996–2016, by Jennifer L. Graham, Guy M. Foster and Ariele R. Kramer, USGS Fact Sheet 2017–3019, U.S. Geological Survey; a work of the United States government in the public domain. The import stopped mid-sentence; its closing sections, the four figures and the four photographs are taken from the fact sheet's PDF. The fact sheet spells MIB "2-methylisoborneal."