White Paper on Oxygen

The Lake Merritt Institute
November, 2003

Background

In May of 1999 the US Environmental Protection Agency placed Lake Merritt on the 303d list, which means it is a body of water whose beneficial uses are impaired. Lake Merritt was listed due to high levels of trash, and low levels of dissolved oxygen. In most cases, placement of this list results in the establishment of Total Maximum Daily Loads (TMDL's) which are regulatory limits on the causes of listing. However in the case of Lake Merritt, the Regional Water Quality Control Board chose to allow Oakland to develop and implement plans to solve the problem without establishment of a TMDL.

Scope of this Paper

This white paper provides information on:

• Sources of oxygen, how it is depleted and why it is important
• Oxygen levels in the Lake
• Known and suspected causes of low oxygen levels in the Lake
• What can be done to improve the situation.

General Information on Oxygen

Sources of Oxygen in the Lake: Oxygen is transferred to water from the air and by photosynthetic activity of plants during daylight. In Lake Merritt, it comes from the vast populations of plant life (plankton, algae and widgeon grass) and from contact with the air. At night, or in places where there is no light, photosynthesis does not occur, and oxygen is not produced. During darkness and especially when they are decomposing,
plants use up oxygen.

Variation: Oxygen levels in the Lake are continually changing in response to daily and seasonal cycles. Peak levels typically occur during mid day when photosynthetic activity is maximum. Minimum levels typically occur just before dawn. The amount of oxygen in the water is also influenced by seasonal factors such as rainfall, runoff, tidal flows, day length, light intensity and the amount of plant life. If water is stratified into separate layers (e.g. top and bottom) oxygen levels can be quite different in these layers.

What Causes Low Oxygen? Oxygen is used up by respiration (breathing) and by chemical reactions. In Lake Merritt oxygen is used up by the respiration of plants and animals, and especially by biological and chemical reactions in the mud at the bottom where leaves and other organic matter are decomposing. Because of this, and because light does not always reach the bottom, oxygen levels at the bottom of the lake are lower than at the surface.

Why is Oxygen Important? Oxygen is necessary for almost all forms of animal life. If deprived of it for even short periods of time, animals will die. The presence of oxygen also means that decomposition will not create odors such as hydrogen sulfide.

How Much is Necessary? Because of its low solubility, oxygen in occurs in water at only a
tiny fraction of the amount in air. Air is 20% oxygen, but when oxygen is dissolved in water, it typically exists at levels from 1 (extremely low) to 20 (very high) milligrams per liter, which is equal to 1 or 20 parts per million (ppm). Various forms of life can endure various levels of oxygen: Some, such as active fish need 5 ppm or more, while others such as clams and mussels can survive at 3 ppm. State water quality standards recommend that oxygen be at least 5 ppm in natural water bodies. But levels below 3 ppm create stress on the aquatic environment and at one to zero, odorous conditions can develop.

Oxygen Levels in the Lake

Most of Lake Merritt is not impaired by low oxygen during most of the time because levels are typically above 5 ppm. However, under certain conditions, some areas of the Lake become impaired.

Alameda County Annual Reports: From 1990 – 1995 the Alameda County Flood Control District published annual reports on water quality in Lake Merritt. Included were the results of weekly or monthly oxygen testing of surface and bottom waters. Briefly summarized, these reports verified that:

• Surface waters contain medium to high levels of oxygen. Typical readings at the Lake center are between 7 and 13 ppm. At the ends of Lake arms, surface levels are typically above 5 ppm. Higher levels are sometimes recorded during plankton blooms. Oxygen levels at the petro-barriers (the black and yellow floating barriers) occasionally drop below 4 ppm.



• Bottom waters are what have caused Lake Merritt to be listed as impaired by EPA. Typical readings at the Lake center are from 5 to 10 ppm (but fell below 3 ppm four times in 1991, and once in 1992). At the ends of Lake arms, bottom levels have typically been below 5 ppm and fell below 3 ppm 14 times in 1991, 9 times in 1992 and 7 times in 1993. Seven incidents in a year may not seem like much, unless you are trying to survive during that incident.



• Plant life can create high levels of oxygen in Lake Merritt during the day, but at night, their respiration can significantly reduce oxygen levels. Also, when plants die off, oxygen is used up by decomposition, and very low levels can occur. In general, excess amounts of seaweed (which dominates in the spring and summer) or plankton (which dominates in the rainy season) can create both high and low oxygen levels.



• Conditions near major storm drain outfalls (outfalls #6, 7, 8, 27, 51, 54, 56 and 60) are the worst of any areas tested in the Lake. These outfalls bring in leaves and other organic materials which use up oxygen in the water as they decompose. The aeration fountains are located in these areas to transfer oxygen from the air to the water.

Continuous Monitoring Probe Data – Lake Center: From June of 2002 until July of 2003, probes measuring temperature, salinity and oxygen were installed by the City of Oakland near the Lake center. They were located about one foot from the surface and one foot from the bottom, and recorded data every 15 minutes. Although this data has not been thoroughly analyzed, the data do reveal that on a weekly basis between June, 2002 and May 2003:

• Mean oxygen levels at the surface were typically above 5, but did drop below 5 four times.
• Mean oxygen levels at the bottom were typically above 5 but did drop below 5 eleven times.

These mean values indicate that most of the time oxygen levels are satisfactory but that there are periods when they become too low. They do not however, represent the stress on animals that occurs when oxygen reaches minimum levels. Looking at weekly minimum data, we find that:

• Oxygen levels at the surface dropped below 3 at least twice
• Oxygen levels at the bottom dropped below 3 at least 15 times
• Oxygen levels at the bottom dropped below 2 at least 6 times
• Oxygen levels at the bottom dropped below 1 at least twice.

Oakland High School Data: Since 1996, students from the Oakland High Environmental Academy have been taking weekly oxygen samples during the school year. There data is particularly valuable because they measure at four stations, not just the Lake Center. In general, water quality in the arms of the Lake is not as good as the Lake center because tidal circulation is not as strong in the upper arms.

Although these data represent grab samples rather than continuous monitoring, they do confirm that, on occasion, very low oxygen levels do occur in the Lake. For example:

• On November 12, 2002, oxygen levels at all four stations were between 1.5 and 3.5.
• On December 10th, 2002, oxygen at the Estuary channel (near the inlet/outlet) and at the Glen Echo station (near the creek inlet) were both measured at 2 at the bottom.
• On December 17th, 2002 oxygen levels at the bottom by the Estuary Channel were 2.5 and 1.5 near the bird islands.

Lighthouse Charter School Data: From June through early December of 2002, the Lighthouse Charter school measured oxygen levels at the surface of the Lake. While most readings were above 5, they were consistently below 5 from September 4th to through October 24th.

Known Causes of Low Oxygen in the Lake
Although more study of the data will provide details, it is known that three factors contribute to low oxygen levels at the bottom of the Lake. These are stratification, decomposition of plant material and restriction of tidal circulation.

Stratification: During and after rainfall, runoff from the 4.650 acre watershed brings fresh water to the Lake. Being less dense, this surface water floats above a layer of tidal salt water at the bottom. When this occurs, especially when the tide gates are closed during high tide for flood control, the bottom layer becomes trapped and loses oxygen due to the decomposition of organic matter carried in with the rain. Such conditions have been documented in the Institute’s March, 2003 “Tidings” newsletter.

Plant Decomposition: Although algae growth in the Lake occurs mainly in the spring, summer and early fall, widgeon grass growth follows a cycle of growth in May and June, followed by die off and decomposition in August and September. As the enormous volume of plant material decomposes, it uses up oxygen. Some of the lowest levels of oxygen have been measured during this period when the widgeon grass is rotting. Large blooms of plankton and algae can also depress oxygen levels when they die off.

Restriction of Tidal Circulation: When the weather forecast includes a 50% or more chance of rain, County personnel prevent high tides from entering the Lake to restrict potential flooding. Surface water drains out during low tides, but since the Lake bottom is 3 – 4 feet below the outfall, the bottom layer of water does not drain well. Deprived of mixing with estuary water during high tides, it becomes lower in oxygen. Such conditions have been documented by the Oakland High Environmental Academy based on 1998 – 1999 data copy available upon request).

What Can Be Done to Improve the Situation?

An earlier report by the Water Quality Task Force listed several important actions which can improve oxygen levels at Lake Merritt. These include:

• Further planning to eliminate sanitary sewer overflows
• A review of the discharges from animal stables at Children’s Fairyland and the police stables
• Modification of tide gate operations (while maintaining flood control)
• Reduction of sediment transported to creeks
• Construction of wetland habitats
• A reduction of leaf / organic matter inflow
• Regular harvesting of widgeon grass and algae
• Minimize pesticide and especially fertilizer input into the Lake
• Reduce the level of bird waste around the Lake and
• Conduct regular water quality testing.

Many of these suggestions cannot be funded by measure DD, which is limited to capital improvements. Following are additional details on those concepts that might be funded by DD:

Aeration Fountains: Currently, three aeration style fountains are installed in Lake Merritt, and a fourth is scheduled to be added in November, 2003. They provide both oxygen and aesthetic benefits, but the increase in oxygen is limited to the local area around the fountain. For example, oxygen levels measured at bottom waters near the bandstand fountain contained above 4 ppm oxygen at a time when levels 20 – 60 or more feet away were at 2 ppm or less. They can provide islands of higher oxygen water when conditions throughout the Lake are poor, and (except for the shallow water intake unit) help to break up stratification of the top and bottom layers.

Aeration Bubblers: These devices have not yet been tried at Lake Merritt. Operating on 110 volts (the fountains use 220) they provide a more economical (but less aesthetic) means of adding oxygen directly to the water. They are commonly used at residential lagoons and in Lakes to provide oxygen and reduce stratification. Small compressors are installed in boxes along the shoreline, and feed air to underwater dispersion devices near the bottom. Capital costs are about half that of fountains and there are no intake screens to clog, but the diffusers may be prone to encrustation by barnacles, mussels, tube worms et cetera. Also, they are least effective in shallow waters. The Institute is currently planning a test bubbler system in conjunction with the fountain to be installed at 18th Street.

Minimize Tide Gate Closure: DD funding of a new flood control station would allow incorporation of state of the art tide gauges and computer modeling to minimize restriction of high tides while maintaining flood control. If decisions on gate closure were made on a 12 hour basis, the gates could be kept open at each tidal cycle unless closure was required for flood control. For more details on tidal flow restriction, refer to the “White Paper on Water Level Control and Tidal Flows” at the Institute website.

Install Bottom Outlet Drains: Currently, water drains from Lake Merritt into tunnels that are 3-4 feet higher than the Lake bottom. This restricts drainage of the lower quality bottom water. Installation of bottom drains as part of the 12th Street re-building under DD would allow removal of this water, which is sometimes very low in oxygen. These drains would empty into the channel on outgoing tides.

Install Storm Drain Filters: By removing organic matter from urban runoff before it
reaches the Lake, storm drain filters can reduce the amount of materials that use up
oxygen. Leaves, fecal material, paper et cetera are trapped in the filter, removed during maintenance and taken to the landfill.

As part of the Water Quality Task Force work, a demonstration storm drain filter (screen) was installed at Lake Merritt 5 about two years ago. An informational brochure describing it is available upon request. The unit has no moving parts, works continuously, does not become clogged, and can be cleaned in about 25 – 30 minutes. Cleaning is required about 4 – 5 times per year and the unit is designed prevent flooding. Oakland’s filter uses a continuous deflection separation (cds) technology and the filter at outfall # 5 prevents items smaller than about 1 x 9 mm. from escaping. It is installed on a drain that is 18 inches in diameter and drains an area of 12 acres. It has worked well since installation (an inspection can be arranged). This type of unit was chosen by the Water Quality Task Force after a study of the various types of stormwater treatment.

Current Plans: Planning is underway for three additional cds type filters; at outfalls, 2, 4 and 45. These filters will be on drains of 24, 24 and 33 inches in diameter and can probably be installed on city land within Lakeside Park.

Remaining Outfalls: With four filters installed, 56 will remain unfiltered, including 7 that are more than 54 inches in diameter or greater. These large storm drains carry more flow, and consequently more oxygen consuming organic matter and trash. However, on these largest filters, tidal flows extend considerably upstream and filters cannot be installed within the tidal zone. The Institute has suggested upstream locations for about eight filters on these larger drains (see memo of August 15, 2003).

Size Limits on Storm Drain Treatment Devices: With the exception of cds type devices and constructed wetlands, storm drain treatment technologies are limited in the size of watershed that they can treat. Subsurface vaults, oil/water separators, media filters, drain inlet devices, vegetated swales and infiltration systems are designed for areas such as shopping centers, parking lots and components of small subdivisions. Devices with such limited capacity are unable to handle the flows to Lake Merritt’s largest storm drains. Because large volumes of water can pass through the cds screen, it is likely that these units can be installed along branches of creeks that feed into our large storm drains.

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