Long-term Increases in Oxygen Depletion in the Bottom Waters of Boulder Basin, Lake Mead, Nevada-Arizona, USA

April 23, 2007

Abstract

LaBounty, J.F. and Burns, N.M. 2007. Long-term increases in oxygen depletion in the bottom waters of Boulder Basin, Lake Mead, Nevada-Arizona, USA. Lake Reserv. Manage. 23:69-82. Long-term changes in the hypolimnetic volumetric oxygen demand (HVOD) of Boulder Basin, Lake Mead were determined from dissolved oxygen profiles collected from 1991 to 2007. HVOD is the rate at which oxygen in a deep layer in contact with the sediments is depleted during the period of thermal and/or chemical stratification. Generally, the rate at which oxygen is depleted is correlated to the amount of organic debris in the hypolimnion and sediments. The sediment oxygen demand reflects historical organic loading, while HVOD is a measure of productivity because of the organic particles settling from above. The lower hypolimnion in Boulder Basin remains relatively stable during the stratification period, enabling the calculation of HVOD in the near-bottom water layer. Small increases and/or decreases that occur in temperature and dissolved oxygen concentrations are detectable. Boulder Basin fully destratifies every other year on average, but mixes only partially in the spring (before May) of the remaining years. The HVOD rates after partial and complete destratification have been assessed separately for 1995-2005. The annual HVOD rate is generally lower the year after partial destratification than after complete destratification due to greater downward transport of oxygen into the hypolimnion. The HVOD of Boulder Basin is variable depending on loading of nutrients and water into the Basin. The rate dropped significantly following commencement of advanced wastewater treatment practices in 1994. The rates then increased 1996-2006 at a rate of approximately 0.75 mg DO/m3/day per year, or about 7% annually. During those years the inputs of nutrients steadily increased. Rates have been dropping from 2005 to present (2007) following further reduction of phosphorus input. A multiple regression analysis revealed that HVOD is significantly positive related to the total phosphorus concentration in Las Vegas Bay, but significantly negative to inflows of Colorado River water. That means HVOD was highest when reservoir water was nutrient-rich and flow rates were low. HVOD should be considered a major tool for monitoring trophic state changes in Boulder Basin.

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New York City DEP

April 9, 2007

New York City DEP is using LakeWatch software to manage the data from monitoring reservoirs that provide 1.3 billion gallons of water to 9 million people every day…

INDUSTRY: Water Supply

PRACTICE: The client is a US government agency tasked with environmental protection. They provide approximately 1.3 billion gallons of high quality drinking water to almost nine million New Yorkers every day — eight million City residents and a million more people in Westchester, Putnam, Orange and Ulster Counties — plus many tourists and commuters who visit the City throughout the year. The source of this water supply is a network of 19 reservoirs in a 1,969 square-mile watershed that extends 125 miles north and west of New York City.

CHALLENGE: It was becoming very difficult to manage the large amounts of water quality data collected from the reservoirs in the Catskill/Delaware system, which accounts for 90% of the daily water consumption.

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South Nevada Water Authority

April 9, 2007

South Nevada Water Authority is using LakeWatch software to provide quantitative results on the water quality of Lake Mead…

INDUSTRY: Water Supply

PRACTICE: The Southern Nevada Water Authority (SNWA) provides a water system for pumping, treating and delivering Colorado River water from Lake Mead to the Las Vegas area.

They are committed to managing the region’s water resources and developing solutions that will ensure adequate future water supplies for the Las Vegas Valley.

CHALLENGE: SNWA comprises of representatives from each of its member agencies: Boulder City, Henderson, Las Vegas, North Las Vegas, Big Bend Water District, Clark County Sanitation District and Las Vegas Valley Water District.

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Environment BOP

April 9, 2007

The Bay of Plenty Regional Council in New Zealand solves the problem of how to determine and maintain the trophic levels of its lakes using LakeWatch software…

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“LakeWatch is a much needed software tool for analyzing water quality data quickly and easily. We can see the data graphically and how it changes over time. This makes it much easier to understand the waterbody dynamics and history. It also means that we can publish interpreted data in a form that the public and policy-makers can understand at a glance.

“We use LakeWatch to give us a quantitative assessment of the trophic (nutrient, clarity, chlorophyll) level of the water. It tells us whether things are getting better or worse and how quickly. In fact, we have now adopted the Burns’s Trophic Level Index for our lakes as a planning standard to determine land use options.

“What we like about LakeWatch is that it is all done at the click of a button where previously using spreadsheets and statistical packages took much longer. We were recently contacted by the head of the Lake Tarawera Residents Association who wanted to know about the lake’s nutrient level – using LakeWatch we were able answer the question while they were still on the phone.”

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Ohio Transportation Engineering Conference Presentation

April 1, 2007

Presentation given at the 2007 Ohio Transportation Engineering Conference, presented by Kirk Beach, Ohio Department of Transportation.


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