The mountains of eastern Tennessee form the rooftop of the Tennessee River watershed. This is the highest ground upon which rainwater falls, ultimately making its way down to the river. The town of Sewanee is on that part of the rooftop known as the Cumberland Plateau. Water falling here has only one desire – to move downhill.

The sublime ease of living at the top of a watershed.

And the journey off the rooftop is a quick one, provided water molecules aren’t caught by thirsty tree roots and evaporated back into the sky. It is estimated that the forests on the Plateau can catch and transpire up to 75% of rain that falls during summer months, thereby creating more clouds and afternoon thunderstorms. The dense sandstones of the plateau make the mountain flat-topped and prevent most water from infiltrating the ground. The water is quickly whisked along small creeks, finally jumping and skipping over cliffs that line the Plateau’s edge.

Water leaving the Cumberland Plateau.

After a brief journey down the steep upper slopes of the Plateau, the water now plunges deep into caverns and passageways of rock formed in the limestones of  the lower reaches of the Plateau. The water has changed chemically and physically. When it first made its way across the Plateau top it was little-altered chemically, save for excess silicon picked up from the quartz grains in the sandstone. Its pH also remained like that of the rain, slight acidic with values around 4 to 5. The journey through limestone caverns has raised its pH to around 8 and large amounts of calcium and magnesium have been released into the water from the dissolving limestones.

Water making its way out of Buggytop Cave near Sewanee (source: Nooga.com).

Towns atop the Plateau also change the chemistry of the water. Winter road salting increases sodium. Rain washing through metal gutters and across automobiles picks up excess copper, lead, nickel, aluminum and other metals. Overloaded streams leaving the hardscape of roads, parking lots and roofs run more turbid than their counterparts in forested ecosystems, having picked up larger quantities of silt and clay. These surfaces also contribute oil, bacteria, grease, litter and plastics of all sizes – even the microscopic. Effluent from sewage treatment plants add their bit to the chemistry of streams through pharmaceuticals, hormones, nitrates and phosphates.

Silty urban runoff during a storm.

Many of these contaminants will make their way all the way down to the Tennessee River via tributaries like the Elk and Duck Rivers. There is a sublime ease in living on a rooftop, unaffected by pollutants found further downstream, but our actions here impact the quality of water others drink downhill. The chemical character of a river is given to it by the sum of what goes on in its watershed.

Wendell Berry once said:

“Do unto those downstream as you would have those upstream do unto you.”

 

Once a part of the Domain (campus) of the University of the South in Sewanee, Tennessee, the Sewanee Natural Bridge was given to the Tennessee Conservation Department in 1976. At more than 100 feet long and 25 feet high this bridge illustrates  the interplay between water and rock structure in the development of a relatively seldom seen feature along the edge of the Cumberland Plateau.

The bridge is the result of erosion by ground water and surface water along vertical joints (geology-speak for cracks) and horizontal partings in the rock. The rock is the Warren Point Sandstone, which forms the prominent cliffs all along the Cumberland Plateau in this area. Formed over 300 million years ago and buried deeply by other rock layers the Warren Point, along with other layers of the Cumberland Plateau, has experienced uplift over the last 5 million years or so. This uplift has lead to the removal by erosion of thousands of feet of rock that once lay atop this layer. This uncovering of the Warren Point released vast amounts of pressure caused by the weight of the overlying rock.  The release in pressure in turn caused the Warren Point to expand and crack along a series of vertical, parallel joints. These joints were natural conduits for the downward flow of rain water, which resulted in the gradual wearing away of the rock along the joints.

The rock in today’s bridge represents the rock between 2 joints that was more resistant to erosion than the surrounding rock. That explains the origin of the thin band of rock in the bridge, but how did the empty space under the bridge form? A powerful clue is found at the base of the main cliff face just uphill from the bridge. Here a spring once flowed with more vigor from a horizontal parting in the sandstone. This groundwater eroded away the rock beneath the bridge.

What the bridge may have looked like in its early stages of formation is hinted at by another nearby location, Widow’s Crack, where a younger natural bridge is forming. Here there is still an active spring eroding away the sandstone under the incipient bridge.

There is only one natural lake in all of Tennessee. Reelfoot Lake in northwest Tennessee was formed by ground subsidence during a series of great earthquakes in 1811/12. All other lakes in the state, including the many dozens on the Cumberland Plateau, are man-made. A quick look at a topographic map or Google Earth image reveals the origins of these lakes. The lakes all have a long, straight edge along part of their shore – a dam (Fig. 2). Lakes on the Plateau tend to be small, especially compared to larger impoundments on the Tennessee River and its tributaries. Lake Cheston in Sewanee, Tennessee is a good example of a typical lake on the Plateau (Figs. 1 & 2).

Even the smallest of Plateau lakes show seasonal fluctuations in chemical and physical properties seen in larger lakes across the temperate zones of the world. As air temperature and solar radiation change through the seasons, lakes go through annual cycles of stratification and mixing.

Summer Stratification

During the summer months the maximum amount of solar radiation beams down upon a lake, warming its waters to a depth of several feet. The summer air above the lake also warms the upper lake portion. Wind blowing across the lake surface mixes the upper lake so that there is a relatively uniform temperature found to a depth of about 6 to 8ft. This layer is referred to as the epilimnion (Fig. 3).

The density of water increases as it gets colder, until a temperature of 4 degrees C is reached. At this point water begins to expand again, becoming less dense. For a lake in summer, this means that the warmest, least dense water will be near the surface (epilimnion) while the coldest, densest water will be at the bottom. This bottom layer is uniformly cold and has no currents to mix it. It is called the hypolimnion. Below the epilimnion and out of reach of the warming effects of the sun and air, the temperature rapidly decreases with increasing depth. This zone is called the metalimnion. Swimmers that drive their toes below a depth of 6-8 ft will feel the distinct cooling within the upper parts of the metalimnion (Figs. 3 and 4).

Because the epilimnion is well mixed by wind, oxygen from the atmosphere has also been dissolved throughout this uppermost lake level. In fact, the epilimnion can often be detected by uniformly high levels of dissolved oxygen (DO)  in the upper 6-8 ft of the lake (Fig. 5).

Along with summer stratification by temperature and DO, there is also stratification based on chemistry in many Plateau lakes. The upper rock layers of the Plateau upon which the lakes are built are made up of sandstones, conglomerates and shales. These contain high amounts of iron (Fe)- and manganese (Mn)- bearing minerals (see previous post titled “Why we’ve pulled the plug on well water on the Cumberland Plateau”). These elements are continuously released by weathering and enter lake waters. In the summer the waters beneath the epilimnion contain little to no DO (Fig. 5). This means that the Fe and Mn can remain suspended within this low-oxygen layer. In the epilimnion there is enough DO to cause the Fe and Mn to oxidize, form a solid and settle to the lake floor. Summer concentrations of Fe and Mn in the hypolimnion typically exceed 10,000 parts per billion (ppb) and 1500 ppb, respectively. In contrast, surface concentrations are normally near 150 ppb Fe and <10 ppb Mn (Fig. 6).

 

Most lakes have some leakage of bottom waters from the base of their dams. As this oxygen-deprived water is exposed to the atmosphere upon leaving the dam base, the Fe and Mn oxidize through the aid of Fe- and Mn – depositing bacteria and precipitate as a red solid on stream floors (Fig. 7). In the early 2000s, the Tennessee Department of Environment and Conservation (TDEC) became concerned that leakage from Plateau dams would bring high levels of the metals far downstream into watersheds, compromising water quality there. Thus they instituted a temporary moratorium on Plateau lake construction. Research I conducted with students showed, however, that stream Fe and Mn concentrations reached normal levels at around 3000 ft downstream of dams.

Some lakes are very shallow and therefore don’t have the range of temperatures to support a fully stratified water body in summer. Brushy Lake in Sewanee is only 8 ft deep and therefore has warm water from surface to base (an epilimnion) (Figs. 8 & 9).

Fall Turnover

During the fall the drop in air temperature and decrease in solar radiation lead to a cooling of the uppermost lake layer. This in turn means that surface waters become more dense than the waters at greater depth. This instability, with the aid of fall winds pushing surface waters to one side of the lake, results in the upper layer of water dropping to the bottom of the lake in what is termed fall turnover. This mixing of the lake destroys the summer stratification and the lake takes on its typical winter profile, with relatively constant temperatures from surface to bottom (Fig. 4).

The diagram above (Fig. 10) shows how decreasing air temperature in the fall causes a decrease in surface water temperature of Lake Cheston, culminating in fall turnover sometime around November 17th. After turnover the lake has a uniform distribution of dissolved oxygen (DO) from surface to bottom (Fig. 5). Notice in the dissolved oxygen graph that winter values of DO are higher overall than in summer. This is because cold water can hold more DO than warm water. Chemical stratification is also destroyed by turnover, with more or less equal values of Fe/Mn throughout the vertical extent of the lake during winter.

Another interesting pattern seen in Plateau-top lakes is zoning by pH. In Sewanee’s Lake Dimmick (also called Day Lake) in 2004, seniors in the Department of Forestry and Geology collected extensive  pH data in the surface waters of the lake (Fig. 11).

 

Notice that the lowest pH values form a “bathtub ring” around the edge of the lake. Rain water in the region typically has pH values between 4.5 and 5. The same values are also found in the soils that cover the Plateau. Thus precipitation falling in the lake’s watershed makes its way into the lake beneath the ground surface as groundwater flow all along the lake shore, with the original pH values essentially unchanged.

It is interesting to compare Plateau lakes to other lakes in the region. For example, is summer stratification similar in the large lakes of the Tennessee River? Below is a graph showing August values of temperature and DO near the dam in Chickamauga Lake near Chattanooga, Tennessee.

 

Notice that the lake lacks the typical summer stratification of temperate zone lakes, with relatively uniform temperature and DO values to a depth of 56 ft. This is because the Tennessee River still flows through its impoundments. Stream flow is turbulent and results in mixing of waters. Thus, while Chickamauga Lake has the appearance of a large lake, the system is still a river with significant mixing.

Andreas has been swimming with only occasional breaks for three hours. I try my best to guide him by keeping my kayak just ahead and to the left in his field of view. With each alternate breath he sees my boat. Between breaths he can just see his fingertips in the pea-green water of the Tennessee River as they execute another freestyle stroke. Except for a woman off to our left, we have separated ourselves from the pack of about one hundred swimmers taking part in this ten mile race. My job is easier. I have time to admire the ranks of trees blanketing the steep slopes to either side of us, the Turkey Vultures scribing circles in the sky high above, and the illuminated sandstone cliffs that form the upper cap of the Cumberland Plateau hundreds of feet above us.

I also have time to think about our swim of the entire 652-mile length of the river last year and the immense amount of water quality data we accumulated then. As it turns out the Tennessee is a fairly clean river. Its levels of pharmaceuticals are lower than what is found in the Rhine River. The heavy metals are low. The nitrates and phosphates are acceptable for a river flanked by extensive fertilizer-dependent agricultural areas. There was one big surprise, however – microplastics. These are pieces of plastic less than 5 mm in diameter that are either the broken down bits of larger plastic or small manufactured beads that are used in some toothpastes and soaps for their abrasive qualities. We analyzed for plastics by pumping 1000 liters of water (same as one cubic meter) through a filter that caught particles in the range of 0.025 mm to 0.5 mm. When we analyzed the first river sample we were so startled by the results that we analyzed it again. What we found was a staggering number: over 17,000 particles per cubic meter. This is the highest concentration of plastic particles ever detected in any river. Looking at the exact same size range a few years earlier in Europe’s Rhine River, Andreas only found 200 particles per cubic meter. The Rhine has ten times as many people living in its watershed compared to the Tennessee. How is this possible?

An answer may lie with another analysis we conducted that looked at the type of plastic found in our samples. Below is a diagram that shows the exact number of each plastic type found in Pickwick Lake, one of the nine reservoirs along the river:

This chart shows that almost half of the plastic found was polyethylene. Most polyethylene is used in light weight packaging and plastic grocery bags. The plastic wrap around produce and the plastic bag it is put into at the checkout counter are likely of this plastic. But how did all this polyethylene get into the river? Although we don’t know for certain, it is highly likely that the majority of it is derived from litter. The samples taken along the length of the Tennessee show roughly the same number of plastic particles from Knoxville, TN to Paducah, KY. Thus we are not dealing only with inputs from particular cities or industrial zones. We as a society are responsible for this plastic load in the river.

Why are microplastics in our waterways an issue? After all, plastic can be swallowed without negative side-effects by animals and people, right? This would be largely true if organisms consumed clean plastic. But plastic particles in a river have been exposed to a host of man-made chemicals that like to stick to their surfaces. Chemicals that include pharmaceuticals, PCBs and heavy metals. In addition to sampling for microplastics during the Tenneswim we also sampled for hundreds of man-made chemicals and heavy metals. What we found was a cocktail of chemicals like anti-seizure medications, blood pressure medications, over a dozen pesticides, pain killers, artificial sweeteners, contrast agents for x-ray and MRI procedures, caffeine, sunscreen ingredients, perfluorinated compounds (PFCs), and a host of other chemicals. That means that each plastic particle acts as a potential transport agent for some of these chemicals. We know that this has caused serious disruption of physiological processes in some river invertebrates (e.g., endocrine disruption), but we don’t yet know about the effects on fish – or humans that drink river water.

As I watch Andreas swim I realize that this means he is carving his way through thousands of plastic particles every few seconds. This also means that the Tennessee River is dumping 32 million plastic particles into the Ohio River every second.

As we emerge from a bend in the river I can see the big, orange buoy that marks the finish line off in the distance. Another 20 minutes and we reach it, Andreas coming in third out of a field of over 100 participants. We leave the water, but the microplastics continue on their journey to the Ohio, then the Mississippi, and ultimately the Gulf of Mexico. There they add to the 9 million tons of plastic entering the oceans annually. Estimates are that if things continue unabated, there will be more plastic particles in the ocean than fish by 2050.

The 2014 water crisis in Flint, Michigan brought forth the realization once again that public water supplies were prone to contamination by dangerous chemicals. In the case of Flint, it was elevated levels of lead that caused alarm. Lead is a toxic metal that can build up in the body over time and can severely affect mental and physical development, especially among children (including lower IQ and increased hyperactivity). When Flint changed its water source from Lake Huron to the more corrosive waters of the Flint River, lead began to leach from the aging plumbing system and entered homes where it was consumed and used for bathing and washing. Some homes showed levels at a staggering 13,000 parts per billion (ppb), well above the Environmental Protection Agency’s (EPA) action level of 15 ppb. The shock continued as other communities discovered elevated lead levels in their water supply systems. The state of Maryland found that seven primary and secondary public schools had high levels of lead in their drinking water. Soon municipalities around the country were scrutinizing their water supplies more closely.

Lead in water doesn’t just get there from corroding pipe systems that are made of lead. Many locations around the world have naturally occurring lead in rock, sediment and soil. If groundwater in these areas is the drinking water source, then lead contamination can be expected. Lead mines often lead to groundwater contamination, as was the case in Picher, Oklahoma, which was declared a superfund site by the EPA. Its more than 1500 residents were mostly bought out by the federal government and the municipality has been a ghost town since its last resident died in 2015. Lead contamination from non-water sources, such as old lead paint in houses, may prove to be an even larger threat that water-borne lead.

What about the quality of water in Sewanee? If you are on the Sewanee Utility District’s (SUD) water supply system, then your water comes from our reservoir lakes (O’Donnell and Jackson). This water has either flowed across the ground or leached through the soil and rock into the reservoirs. Happily, the substrate in the area is very low in metals that are of concern in drinking water, including lead. After cleaning and chlorination at the filtration plant the water is sent on its way through pipes to town buildings. The pipes are made of various metals and plastics that could contribute some of their constituent materials to the water. One of my environmental science classes recently had tap water analyzed that was sampled from 24 University buildings (no private homes or businesses were sampled). The results indicate very low concentrations of 59 inorganic elements (metals) commonly found in tap water. Not only were metals well below the maximum allowable levels for drinking water as outlined by the EPA, but some metals of particular concern, like lead, were at such low levels that they could barely be detected.

Iron, which is classified by EPA as a secondary contaminant, can be a nuisance in well and lake water on the Cumberland Plateau, staining laundry and plumbing fixtures pink. It is found at such low levels in our tap water that one would have to drink about 180 liters of it to get the same amount present in a single typical, iron-bearing children’s vitamin. This is the most extensive sampling ever done for inorganic elements in tap water on the campus (note that other contaminants and biological components were not analyzed). The results were shared with the Sewanee Utility District and will contribute to a much more complete picture of Sewanee’s drinking water quality. Bottoms up!

 

Before we successfully built the first modern reservoirs in the Sewanee area in the 1950’s, well water was an important component of our water supply along with the few perennial springs in the area. The first wells were hand-dug, extending down through the soil and then through as much of the weathered bedrock as the well diggers could manage. These old wells (like the one pictured above) typically ranged in depth from 20 to 35 ft. A rare glimpse into a well behind and old residence on University Ave. and in the woods on “Billy Goat Hill” in Sewanee reveal that the structures were rock-lined and between 3 and 4 feet in diameter (see photo below).

The rock lining would have prevented collapse of soil into the well and presumably did not extend into the bedrock to any great distance. Upon completion, the well was capped with a lid and a pipe moved water from the well bottom to the surface via a hand-operated pump. These old wells were problematic for two main reasons. The first is that their shallow nature meant they would run dry when the water table dropped due to drought or seasonal decline during dry periods of the year (summer and fall). The second issue is that they were not sealed off (or cased-off in well driller speak) from the surface. That meant that surface water, along with any contaminants it carried, could easily infiltrate the well. During heavy rains one can sometimes hear water pouring from the surrounding soil down into these old wells. This was not a good arrangement if a privy or concentration of animals was near the well.

Today wells are drilled with much greater ease using compressed air drilling techniques, so that a well over 100 ft. deep, penetrating hard sandstone, can be completed in just a few hours.

These wells are cased off for the first 21 ft., so that there is little chance of surface water  entering the well. Below are field notes showing the construction of a modern well in Sewanee:

The field notes show that a 8.5 inch diameter hole is first drilled through the soil and then into the bedrock to a depth of 21 ft. Steel casing (a steel tube) of 7 inches diameter is then placed in the larger hole. The space around the outside of the steel casing is then filled with a bentonite slurry. Bentonite is a swelling clay and forms a tight, waterproof seal around the steel casing. Now a 6 inch diameter hole is drilled to the desired depth below the water table. In most cases the borehole is left unlined, with the firmness of the rock preventing the hole from collapsing.

Why then, with this efficient method of providing safe access to groundwater, have most people on the Plateau chosen to opt for reservoir water as their drinking water source? The answer lies with issues related to both water quantity and quality. A quick glance at the water well drilling records maintained by the state of Tennessee (or in conversations with well drillers) reveals that the average yield of wells on the Plateau is between 3 and 5 gallons per minute. This might prove satisfactory if one has the ability to store the well water in a large tank, but this does not suffice for a household pumping the water directly from the well into the house. The reason for these low yields is primarily due to the fact that the sandstones and shales of the upper Cumberland Plateau have relatively low permeability. Water travels through these layers mainly along fractures and one must be fortunate enough to have a well that intersects these fractures. Predicting where these fractures are found in the subsurface is not possible. Compare these yields to those from wells drilled in the valley surrounding the Plateau, where wells often produce from 80 to 100 gallons per minute. This increased production is due to the fact that valley wells are drilled into very permeable limestone that is honeycombed with caves and passageways filled with water. Drilling deeper on the Plateau down into the limestone will not increase yield, since caves and associated passageways do not extend beneath the sandstone cap of the Plateau.

Geologic cross section of the Cumberland Plateau near Sewanee. Rocks above the Raccoon Mountain Formation are relatively impermeable sandstones and shales. Rocks below this layer are chiefly limestones with good permeability given to them by caves and passageways only in areas not under the sandstone cap.

 

The second problem with well water on the Plateau has to do with reduced quality because of high levels of iron. Now, iron is not considered a primary contaminant in drinking water by the Environmental Protection Agency (EPA). Rather, it is classified as a secondary, or nuisance contaminant, causing stains on bathroom fixtures, turning white laundry pink, and clogging water filters. The general rule of thumb is that the greater the depth from which water is drawn from a well, the higher the iron content will be. This is due to the fact that deeper groundwater contains less dissolved oxygen than shallow water. Dissolved oxygen makes the iron precipitate out of solution (turn solid) and settle out of the water. The same trend is seen in spring water. Shallow springs (like Tremlett Spring in Abbo’s Alley or Hat Rock Spring) are very low in iron. These springs emanate from the Sewanee Conglomerate (see cross section above), which sits on top of the Plateau in the Sewanee area. Deeper springs that emanate from the underlying Warren Point Sandstone have much higher levels of iron and have historically been called chalybeate.

The ultimate source of all the iron in the groundwater is from the weathering of iron-bearing minerals in the sandstone.

So, due to low yields and high iron, the vast majority of people on the Plateau have opted for reservoir water (if it was available to them) instead of well water.

Big Creek Gulf Hydrology

One of the best places to witness karst hydrology and beautiful scenery is within the Savage Gulf State Natural Area near Beersheba Springs, Tennessee. Here, three deep gulfs cut by stream erosion dissect the Cumberland Plateau like the imprint of a giant crow’s foot. Big Creek has carved out one of these gulfs and is best accessed via the Stone Door.

From here one passes through a large joint in the Warren Point Sandstone to drop more than 600 feet along the Big Creek Gulf Trail (BCG trail on map) to the rocky bed of Big Creek. About 100 yards downstream from where the BCG trail meets the creek, several large springs emerge from the hillside on the other side of the stream. This is also the contact between two major rock layers of Mississippian age – the Bangor Formation above and the Hartselle Formation below.

The occurrence of springs at this stratigraphic level is no coincidence. The overlying Bangor Formation is mainly limestone that has been heavily chemically weathered and contains many cave passageways. The underlying Hartselle Formation, on the other hand, is a calcite-cemented, quartz sandstone with no karst development. It forms an impermeable boundary to groundwater flow and the downward development of caves within the Bangor.

Moving upstream from the springs the bed of Big Creek is dry. The imbricated and polished rocks of the bed, along with high water debris lines and bent over trees clearly indicate that there is a vigorous flow of water here at certain times.

Further upstream at both Ranger Falls and “The Sink” (see map above), one is greeted with the sound of flowing water. Both places mark the top of the Bangor Formation where it contacts the overlying Pennington Formation. Here water flowing across the surface of the relatively impermeable Pennington quickly disappears into the caverns of the Bangor.

Because the Pennington Formation contains numerous shale layers interbedded with limestones and dolomite, it does not conduct groundwater to the extent that the Bangor does. It also has far fewer and smaller caves. Upstream from “The Sink” Big Creek flows above ground and, for the hiker, culminates at Greeter Falls. During heavy or prolonged rains, the karst system of the Bangor Formation is overwhelmed by water and Big Creek is characterized by a continuous flow along its entire length. Thus the karst hydrology is best detected during relatively dry periods.