The Green Shadow on The Lake

Lake Mead has exhibited several algal blooms during the past decade. The most widespread bloom in 2001 has been considered very unusual for the area. "The bloom was so extensive that it spread throughout the lower Colorado River system … and even into reservoirs in the Southland region as far south as San Diego, a distance of about 400 miles," said Mic Stewart, the water quality manager for the Metropolitan Water District of Southern California.

The large algae bloom at Lake Mead in 2001 (photo left). The photo at right depicts the lake with its normal water color. photo courtesy of H2O University. (http://www.h2ouniversity.org/html/library_water_facts_algae.html

 Although authorities said the sheen of green algae on Lake Mead was non-toxic and it didn’t impact the water quality and aquatic habitat, it had a great effect on recreational business of the area with a six percent drop in the number of tourists between May and July.

The causes of the 2001 algae bloom were not completely understood; but the appearance of those blooms indicates the vulnerability of the Lake’s environment to nutrient loads (phosphorus and nitrogen). Phosphorus in the lake is provided by discharged treated wastewater, urban runoff, and wildlife; however, there is no reliable information on the magnitude of nutrients from different resources. The three treatment plants - Las Vegas, Clark County and Henderson - that dump treated wastewater into Lake Mead through Las Vegas Wash (a natural wash which drains most of the Las Vegas Valley’s excess water into Lake Mead), voluntarily remove more than 90% of phosphorus from all discharge in advance, but there is less control on the sediment load and bacterial count carried by urban runoff and stormwater.    

Causes of the bloom

It is impossible to pinpoint what exactly caused the algae bloom in Lake Mead, but it can be said that a combination of physical, chemical and biological changes over decades have influenced this event. However, most of the experts have consensus on the limiting impact of nutrient input from dumping wastewater and, most importantly, urban runoff into Lake Mead. Located in the middle of the desert, unlike Los Angeles and San Francisco, Las Vegas does not have access to the Pacific Ocean for wastewater disposal. The excess water has no other option than to return to Lake Mead. 

Urban Runoff

Excess water from lawns, washing cars and hosing down driveways washes away fertilizers (rich in phosphorus), pet waste and household chemicals, draining them directly to Lake Mead. Although urban runoff makes up for a very small portion of the Wash flow into Lake Mead, it is a concern since it carries various pollutants and nutrients from fertilizers and from the urban environment. The polluted water then is sent directly to Las Vegas Wash, without being treated. Other sources of pollution and nutrients include livestock waste and faulty septic tanks.

Excess water from lawns is drained directly to Lake Mead, without being treated. photo courtesy of Las Vegas Wash Coordination Committee

Sprawl is considered one of the main causes that raise the amount of pollutants in Lake Mead. The population in the Las Vegas Valley has been growing fast since the 1930s when the construction of the Hoover Dam kicked off the first population boom in Las Vegas. Each new subdivision in Southern Nevada results in more wastewater and runoff drains into Lake Mead.

Las Vegas Wash

Urban runoff and treated wastewater is discharged into the lake through Las Vegas Wash. The Wash is a 12-mile “urban” river that flows between the city of Las Vegas and Lake Mead. Once known as an intermittent flow, the Wash became a perennial stream as a result of the discharge of treated flows from the Valley’s wastewater facilities, which began in the 1950s. This river owes its flow to the urban population living in Southern Nevada. The Wash carries more than 150 million gallons of water to Lake Mead on a daily basis, which accounts for two percent of all the flows into the Lake. This river is the Valley’s sole drainage to Lake Mead and is comprised of four major flows: urban runoff, shallow ground water, reclaimed water and stormwater. Urban runoff and shallow groundwater seepage build the base flow; treated wastewater is added later along the lower Wash and considered the dominant flow therein. It’s not surprising that the Wash’s flow (the amount of water that is discharged into the river) has been proportionally increased with Southern Nevada’s population growth. The Wash plays an important role in extending the limited water supply for Southern Nevada residents by returning about 50 billion gallons of water a year to Lake Mead. The Wash also serves an important role as a wildlife habitat and a recreational area, featuring trails and wildlife viewing sites for the residents.

The map of Las Vegas Wash, photo courtesy of Las Vegas Wash Coordination Committee

Erosion and wetlands deterioration

Unfortunately, the increased erosion since the 80s has washed away a big portion of wetlands and has diminished their size from 2000 acres to 200 acres, reducing their performance as water filters. Wetlands along the Wash have been acting as kidneys, filtering water from oil and residues and absorbing excessive nutrients. These oases also host more than 700 species of birds, reptiles and other animals, making the Wash one of the most unique natural resources of Southern Nevada. Erosion is considered the biggest threat to the existence of Las Vegas Wash and one of the factors that stimulates algal growth.   

 

Adverse consequences

Algal blooms are considered threats to aquatic environments. Green algae are not toxic by its nature; the problem appears when they die and start decaying. “As they die, they consume oxygen and if the oxygen levels drop below a certain level, it could be bad for fish” said Tom Porta, chief of State Bureau of Water Quality Planning. Fish require at least five milligrams of oxygen per liter of water. Scientists fear the green algae are replaced by potentially toxic blue-green algae, which are toxic and harmful to fish. Plus, decaying algae provides a suitable environment for pathogen growth by increasing their hosts.

Algal blooms have adverse impacts on recreational businesses around the lake. Clear water is more pleasing for swimming and other recreational activities. Also, the value of lakefront properties is, to a great extent, determined by the clarity of the water. A turbid, smelly lake can cause a drop in real state value.

     

Also, over 17 million users from the downstream states rely on the Colorado River outflow from Lake Mead as a water resource. Any water quality issues in the Lake affect not only the Las Vegas users but all the downstream consumers as well.

Since 1998 many water coalitions have tried to tackle the water quality issue in Lake Mead. The SNWA has spent millions of dollar to modify the water quality by upgrading wastewater treatment plant, promoting water conservation and water recycling and conducting numerous studies.  (Many of the SNWA’s conservation programs such as the turfgrass replacement plan could contribute to less chemicals and fertilizer in runoff.)   

In spite of all the conducted measures to decrease nutrients and chemical loads in Lake Mead, still part of the lake display recurring algal blooms. The constant water quality issue can raise an important question: Is it worth it to increase the water supply by two percent (by draining the city’s runoff, and effluent water into Lake Mead), with the expense of raising water quality issues for millions of people and degrading the valuable ecosystem?