07/17/17

Long-Term Trends in Transparency

Transparency vs. Precipitation

Climate Change

Proposed Restoration Goal

Analysis of Temperature & Dissolved Oxygen Profiles

Phosphorus Data

Potential Remedy for Sediment Phosphorus Releases

References

Friends of White Pond - Ponderings 2015

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Following is a summary of the historical transparency and water level data collected by the Friends of White Pond between 1987 and 2013.

Supports Concord's current efforts to develop a management plan to restore & protect water quality and ecology.

The Concord Board of Selectmen Has Scheduled a Town Forum on White Pond, Harvey Wheeler Center, 6:45 pm, January 21, 2015.

Water Level Fluctuations

Historical water levels varied over a range of approximately 60 inches ( 5 feet ), relative to the maximum depth of ~55 feet.

Variations in water levels are highly correlated with variations in precipitation averaged over 2-3 year period.

The lag time likely reflects the large volume of groundwater storage and slow discharge rates from the aquifer to downstream waterbodies.

The recent low water levels are explained by low precipitation, although increased pumping from the town well could also be a factor.

Water levels in Walden Pond are also very low, although time series data are not available.

The pond is more susceptable to effects of runoff and erosion from shoreline areas during periods of high rainfall and water level.

When water levels are low, some of the eroded materials from the watershed could be trapped in the beach area and not reach the pond.

When water levels are high, watershed loads are more likely to reach the pond and wave action could contribute to erosion of the banks.

When water levels are lower, the assimilative capacity of the pond to handle a given phosphorus load is lower.

The rate of oxygen depletion in the bottom waters would also tend to be higher in shallower years because of the smaller volume.

This could have adverse impacts on the fish habitat and increase the rate of phosphorus release from the pond bottom sediments.

Long-Term Trends in Transparency & Turbidity

Water clarity apparently improved over the 1987-2005 period (Secchi depth increased, turbidity decreased).

The improvements are possibly related to management measures taken after the 1987-1988 baseline study.

Measures included construction of an infiltration basin at the base of the boat ramp, improvements to shoreline septic systems, etc.

The improving trend was reversed around 2005. Secchi depths were generally below the long-term median (~ 6 meters) in 2009-2014

The lower water clarity in recent years is consistent with increasing erosion at several locations around the shoreline and plugging

of the infiltration basin due to inadequate maintenance.

( see plans & photos )

Water clarity improved slightly in 2015-2017. This could reflect lower precipitation rates and repair of the infiltration basin in 2016.

Transparency & Turbidity vs. Antecedent Precipitation

The charts above show that significant decreases in water clarity were associated with high precipitation in May of 2006.

This was the infamous "Mother's Day Storm" that totaled about 7 inches on May 12-15.

Wiki Description

NOAA Record

Secchi depth decreased from 10.3 meters (~historical high) on May 7, to 8.0 meters on May 25, and 2.6 meters on June 30 (~historical low).

A single erosion event associated with extreme rainfall could have a long-term impact on both the watershed and the pond.

Unless repaired, gullies created in the steep shoreline banks would be more susceptable to erosion in subsequent smaller storms.

Nutrient loads discharged into the pond during runoff events are stored and recycled within the pond, so we would not expect a 1/1 relationship between precipitation and clarity in each year.

Climate Change

National Climatic Data Center

http://www.ncdc.noaa.gov/monitoring-references/maps/us-climate-divisions.php

Long-term climate records indicate increasing trends in air temperature and precipitation, particularly after 1960.

Increases in air (hence water) temperature would have an adverse impact on the cold-water fish habitat and increase algal growth rates.

By increasing evapotranspiration from the watershed, warmer conditions would to tend to reduce groundwater levels, pond inflows,

and pond water levels, especially in drier years. Pumping rate from the town well could also increase because of increased demand.

Increases in the mean and variability in precipitation would trigger increases in runoff, erosion, and higher water levels.

These results point to the importance of controlling erosion from the adjacent watershed to maintain existing water quality, particularly given the expected increase in the lake user population associated with the Bruce Freeman Railtral and the obvious evidence of erosion at several locations around the pond.

Restoration Goal

Clearly-defined numerical goals are needed to track future progress in restoring White Pond to its highest potential.

Restoration goals for high-quality water bodies are typically based upon anti-degradation concepts.

The objective is to provide future water quality that is at least as good as that observed historically (1999 - 2005 ).

Summer-average Secchi Depths in Walden Pond ranged from ~ 4 to 6 meters in 1997-1999.

Transparency provides a practical surrogate for other water quality problems triggered by nutrient and sediment loads.

An analogous restoration goal for fish habitat could be developed based upon the historical temperature & oxygen data.

USEPA National Lakes Assessment, 2007, Random Sample of 1092 Lakes ( 106 in Northeast, EPA Regions 1 & 2 )

http://water.epa.gov/type/lakes/lakessurvey_index.cfm

Cape Cod National Seashore Kettle Ponds (April Data).

http://www.nps.gov/caco/naturescience/upload/Pondatlasfinal.pdf

White Pond, June-August Means, 1987-2014

Percentile Relative to Other Lakes

Depth > 10 meters

All Lakes

Value

Description

Observed in Years

USA

Northeast

USA

Northeast

>7.5 m

Proposed Long-term Goal

2003 - 2005

90%

91%

97%

96%

> 6 m

Proposed Interim Goal

1999 - 2005

80%

74%

93%

89%

4.5 m

Min ~ "Permissable Load"

1987, 2006, 2009, 201

66%

45%

88%

72%

Analysis of Temperature & Dissolved Oxygen Profiles

Vertical temperature and dissolved oxygen profiles are important diagnostic tools. They provide indicators of long-term changes in water quality related to eutrophication and climate change. They also provide a basis for evaluating cold-water and warm-water fish habitat by comparisions with relevant water quality criteria. Depletion of oxygen from the bottom waters is a signal of increased algal productivity and recycling of phosphorus from the bottom sediments. Increasing trends in surface water temperature have reduced fish habitat and increased the risk of blue-green algal blooms. Interpretation of profiles is complicated because of the multiple dimensions involved (depth, season, & year). The profile data are also influenced by short-term and long-term variations in weather, including air temperature, wind velocity, and solar radiation.

Comparisions of dissolved oxygen & temperature profiles collected in 2011-2017 are displayed for separate months. This provides a basis for evaluating long-term trends while normalizing for seasonal effects.

Detailed results are shown in the attached figures for:

June

July

August

The following figure shows vertical dissolved oxygen profiles collected in June of 2011-2017. Dissolved oxygen levels below 30 feet were significantly lower in 2014-2017, as compared with 2011-2013. This is a symptom of increasing decomposition of organic material derived from algal growth and accumulating in the bottom sediments.

Data from 2006-2017 displayed in a 3-Dimensional format below. The vertical axis is water depth in feet. The horizontal axis is sampling date. The colors reflect different levels of dissolved oxygen, temperature, and fish habitat zones. The Massachusetts water quality criteria for cold-water fish are a maximum temperature of 20 deg-C and minimum dissolved oxygen concentration of 6 ppm. The criteria for warm-water fish are 28.3 deg-C and 5 ppm, respectively. Blue symbols represent depth intervals and dates when the oxygen temperature values conform to these criteria for each fish category.

Criteria reference:

http://www.mass.gov/eea/docs/dep/service/regulations/314cmr04.pdf

The plots show that:

- Dissolved oxygen in the lower waters has decreased in recent years.

- Temperatures have increased in both the bottom and surface waters.

- Fish habitat has decreased, especially for cold-water fish.

PDF Version

Results for each profile can be summarized as depth intervals conforming to various criteria. This allows display of the data in a simpler time-series format. The following charts show long-term trends in fish habitat, dissolved oxygen, and temperature based upon profiles collected in June-August of 2006-2016. A number of factors may contribute to these trends, including accumulation and recycling of nutrients within the pond, climate change triggering increasing water temperatures, decreases in water depth, and antecedent weather conditions.

The fish habitat zone is defined as the depth interval conforming to the oxygen and temperature criteria for each fish category. For example, a value of 20 feet indicates that 20 out of the total ~50 feet represents suitable habitat.The habitat zone varies seasonally because of increasing temperatures and decreasing oxygen concentrations between June and August. While fish may survive for brief periods outside of the habitat zones, reproduction would be inhibited.

The June-August average habitat zone for cold-water fish decreased from ~28 feet in 2006 to ~12 feet in 2016. The monthly average habitat zone for cold-water fish reached a historic low level of 4 feet in August of 2016. This has important implications for fisheries management. The routine practice of trout stocking should be reconsidered in light of the declining trend in habitat. Unless the stocked trout are harvested by fishermen, they would represent a significant source of nutrients that further increase eutrophication of the pond. Death of unharvested fish triggered by stressful habitat (particularly in August), settling of the fish carcass to the bottom waters, and subsequent decomposition would further aggravate the low dissolved oxygen levels and increase phosphorus recycling.

The warm-water fish habitat zone is larger than the cold-water zone because of greater tolerance for high temperatures and low dissolved oxygen concentrations. The June-August habitat zone for warm-water fish decreased from ~44 feet in 2006 to ~35 feet in 2016. The monthly average habitat zone for warm-water fish reached a historic low level of 32 feet in August of 2016.

Trends in oxygen depletion are expressed as depth intervals with oxygen levels below 5 ppm and 2 ppm. Both exhibited increasing trends over the 2006-2016 period.

The 5 ppm threshold reflects habitat unsuitable for warm-water fish. This seasonal-average depth interval below 5 ppm increased from ~9 feet in 2006 to ~18 feet in 2016. The monthly-average interval reached an historic maximum of 22 feet in July-August of 2016.

Dissolved oxygen levels below 2 ppm reflect conditions lethal to most fish and an increased risk of phosphorus recycling from the bottom sediments. An increase in the depth interval with DO below 2 ppm implies an expansion of the bottom sediment area releasing phosphorus to the water column. The seasonal-average depth interval with DO below 2 ppm increased from ~6 feet in 2006 to ~14 feet in 2016. The monthly-average depth interval reached an historic maximum of 16 feet in August 2014, June-August 2015, and July 2016.

Trends in temperature are expressed as depth intervals with temperatures exceeding 20 and 25 deg-C. Both exhibited increasing trends over the 2006-2016 period. It is likely that these trends reflect increasing air temperatures and declining water depths.

The 20 deg temperature threshold reflects habitat unsuitable for cold-water fish. This seasonal-average depth interval exceeding 20 deg-C increased from ~14 feet in 2006 to ~23 feet in 2016. The monthly-average depth interval reached an historical maximum of 28 feet in August 2016.

The 25 deg threshold reflects conditions conducive to high algal growth rates and increased dominance of blue-green algae over other species. The seasonal-average depth interval exceeding 25 deg C increased from 6 feet in 2006 to 13 feet in 2016. The monthly-average depth interval reached an historic maximum of 29 feet in August 2016.

It is likely that the relatively hot and dry summer of 2016 contributed to the elevated temperatures measured in the Pond.

Phosphorus Data

Phosphorus data collected since 2000 are summarized in the table below. This includes analyses of samples collected by the Friends of White Pond and analyzed by Upstate Freshwater Institute (UFI), as well as data collected by ESS under contract with the Town of Concord. Because of the high varibility in phosphorus measurements, at least three vertical profiles are typically collected to adequately assess trophic status and long-term trends. While the data are limited, phosphorus levels in 2014-2016 were higher than those measured in previous years, particularly at depths below 15 feet, where average concentrations ranged from 3 to 38 ppb in 2000-2013, as compared with 21 to 158 ppb in 2014-2016. This is a symptom of increased phosphorus release from the bottom sediments. Addressing this nutrient source, as well as watershed runoff and erosion, is required to restore the Pond.

Potential Remedy for Sediment Phosphorus Releases

A recent issue of Lake and Reservoir Management, a publication of the North American Lake Management Society, describes experience in utilizing chemical treatment to address sediment phosphorus sources.

One article is an overview of global experience.

Wagner (2017)

Another describes applications to ponds on Cape Cod.

Wagner et al. (2017)

Preliminary cost estimates for applying this technology to White Pond based upon the Wagner et al article are described below.

Alum (aluminum sulfate) is a chemical that is widely used for treating drinking water, as well as for treating lake sediments to control phosphorus releases. Cape Cod ponds treated with alum ranged in total size from 19 to 731 acres, as compared with White Pond area of 39 acres. Treatment is typically targeted to the deeper water zones where oxygen is depleted and sediment phosphorus levels are relatively high. Treated areas of Cape Cod ponds ranged from 12 to 366 acres.

In a recent report to the Town of Concord, ESS measured sediment phosphorus concentrations ranging from 157 to 2,740 mg/kg at several locations throughout the pond (see map below). The highest values were located in the three deep zones (2,300 - 2,740 mg/kg), as compared with a range of 157 to 1,520 mg/kg at other locations. Based upon August 2016 oxygen profiles, ESS also estimated that the area of anoxic sediments in the three deep zones was approximately 8.2 acres. If this were used as a basis for designing an alum treatment of White Pond, the treated area would be small (8.2 acres), relative to the treated areas in Cape Cod ponds (12 - 366 acres).

The cost of the Cape Cod treatments in 2016 dollars averaged $150 per gram of alumimum applied per square meter per hectare of treated area. Aluminum doses ranged from 10 to 100 grams per square meter. If these cost factors were applied to an area of 8.2 acres, the cost for treating White Pond would range from $4,980 to $49,798, or 5 to 50 K$. A feasibility study would be required to provide more accurate estimates for the appropriate treatment area and Aluminum dose. Treatments typically require a state permit.