alternative futures for the city of la paz,...

6 • May/June 2004 • Southwest Hydrology

O N T H E G R O U N D

Alternative Futures for the City of La Paz, MexicoCaroline Dragoo – University of Arizona and Robert Faris – Harvard University

La Paz, Mexico is working hard not to turn into Cabo San Lucas, its neighbor to the south that has been so overrun with tourism that the city’s charm has been compromised. La Paz’s 190,000 residents prefer to “keep it Mexican” and strive for sustainable growth. The city sits on a lagoon within a bay on the peninsula of Baja California (see map). It is growing so rapidly that most current city planning consists of reacting to changes. In order to gain some control of future development and ecotourism, local officials needed insight into possible scenarios for growth. The International Community Foundation and the Mexican Foundation for Environmental Education have funded a project in collaboration with the University of Arizona and Harvard University to create scenarios of alternative futures.

The project focuses on modeling the interactions among hydrology, tourism, economy, and marine ecosystems (see diagram, p. 7). Since the processes are interconnected, the programs that model

these processes will be linked. The goal is to create an interconnected model to predict how growth will impact each system twenty years in the future. For example, if tourism is projected to increase, population also will increase, requiring more water, increasing sewage releases into the bay, in turn affecting marine ecology and impacting the fishing industry.

Scenarios of alternative futures will model several supply variables that determine the amount and location of land assumed to be available for development, as well as demand variables that reflect the amount of projected population growth and tourism growth by category (see table, p. 7). The three supply variables are: ALL, which assumes all land is open for development, including currently protected

land; LAW, which assumes that currently protected land will remain protected; and CONSERVATION, which assumes there will be additional restrictions on development in areas such as the 100-year flood plain, regions with high biodiversity habitat, and high-quality view corridors. Likewise, there are three demand variables: BASE, which assumes that the population will grow as forecast for each of the three socio-economic levels and that tourism will grow as forecast for each of the three tourism types; LOW-INCOME, which assumes the lowest socio-economic group will grow twice as fast as the baseline forecast and that all other population and tourism groups will grow as forecast; and HIGH, which assumes that tourism and population growth for all groups will be twice as fast as forecast.

Surface and groundwater processes both impact and are impacted by the growth scenarios. The region is hurricane-prone, with a 50 percent annual chance of occurrence. In 1976, Hurricane Liza struck La Paz, delivering more than 7 inches of rain in one day, approximately the average annual rainfall for the city. In the model being developed, a hurricane with Liza’s magnitude will be forecast for each of the scenarios. Some scenarios assume the city will have built flood control structures, others do not.

A MODFLOW groundwater flow model

Location of La Paz, Mexico and inset of Landsat image

May/June 2004 • Southwest Hydrology • 7

is being developed to determine the impacts of demand on the aquifer and resulting saltwater intrusion. This model is expanded from other models and studies developed by the Universidad Autónoma de Baja California Sur (UABCS). Future demands, as determined by the nine supply and demand scenarios, will be imposed on existing 2004 conditions. For example, a 50 percent increase in domestic use and agriculture by 2014 will enlarge the cones of depression around the main well field and increase salt water intrusion. For each scenario, a map will show depth to water table as well as the approximate location of the saltwater intrusion. The results will be displayed for 10 and 20 years in the future.

In May 2004, final results of all modeled scenarios will be reported to government decision makers and regional stakeholders. The goal is for the entire community to learn about the impacts of growth so they can make necessary preparations for likely, or even worst-case scenarios.

For more information, contact Caroline Dragoo at [email protected].

Demand Alternatives

Supp

ly A

ltern

ativ

es BASE LOW-INCOME HIGH

ALLLeast implementation High environmental

degradation & budget pressure

Most spreadMost impactMost demand on services

LAW Public policy assumption Probable outcome under moderate conditions

Implementation pressure high

CONSERVATION Most ‘sustainable’ Social pressure on conservation policies

High pressure & hardest to implement

Growth scenarios to be modeled in the Alternative Futures Study (see text for explanation of categories).

Commercial andartisanal fishing

Population bysocio-economic groupTourism by sub-market

HurricanesNormalRainfall

Demand fornew land use

Runoff

New land useallocations

Terrestrialecology

Visualquality

Speciesrichness

Aquiferrecharge

Sewercapacity

Flooding

Groundwater

Fishproduction

Marineecology

Employmentand income

Policies andexogenous inputs

Feedback linkages

SalineIntrusion

Landvalue

Discharge

Alternativescenarios, policies

and plans

Existingland use

Wateruse

Processmodels

=

=

Near-shorepollution

Systems involved in the Alternative Futures study

8 • May/June 2004 • Southwest Hydrology

Hydrogeologic Investigation for City of Phoenix SR 85 Landfill Greg Bushner, R.G., Pascal Hinnen, P.E., and Andrew Messer, R.G. – URS Corporation

The State Route 85 Landfill was planned to meet future solid waste management needs for the city of Phoenix, and is expected to be operational in 2005. URS Corporation used criteria-based GIS analysis to select the location of the landfill, and conducted an extensive hydrogeologic investigation in support of permit applications for the proposed site. It is in the Gila Bend groundwater basin in southwestern Maricopa County, 17 miles south of Interstate 10 and one-half mile west of SR 85 (see map).

The landfill site covers 2,652 acres and the surrounding land is used for agriculture, commercial, and residential purposes. Farming will continue at the site during landfill operation. Approximately 2,050 acres will be permitted for solid waste disposal; the remainder will be used for ancillary facilities, 350- to 500-foot buffer zones around the perimeter, and stormwater management structures, including a 160-acre stormwater retention area. The natural surface drainage is west-southwest, toward the Gila River. Maximum depth of the landfill will

range from 60 feet on the western side to 120 feet on the east. The landfill will be a maximum height of 150 feet above existing grade and shaped to minimize visual impacts to the surrounding landscape. Berms and landscaping will be used to screen land-filling operations as each phase is constructed.

The main focus of the hydrogeologic characterization was to demonstrate that the landfill would not cause or contribute to a violation of Arizona’s Aquifer Water Quality Standards (AWQS). The landfill will be constructed with a composite liner and a leachate collection and recovery system (LCRS) to protect groundwater. From bottom to top, the liner system consists of a geosynthetic clay layer, a 60-mil HDPE liner, a geonet drainage layer, a non-woven geotextile to prevent

May/June 2004 • Southwest Hydrology • 9

soil clogging of the geonet drainage layer, and a 2-foot soil layer to prevent liner damage. The Hydrologic Evaluation of Landfill Performance (HELP) computer model was used to demonstrate liner effectiveness in preventing fluid migration. The LCRS was designed to collect and remove leachate, maintaining less than one foot of head over the liner system during a 25-year, 24-hour storm event.

The landfill site was selected to optimize factors that contribute to groundwater protection. Infiltration of direct precipitation will be negligible, as indicated by the water balance resulting from the annual precipitation rate minus the evaporation rate. The relatively thick vadose zone allows the processes of natural attenuation to occur. Field investigations included 52 soil borings to depths of 50 to 150 feet. Samples were analyzed for particle size distribution, Atterburg limits, Proctor compaction, and moisture content.

Monitor wells will be installed to analyze groundwater flow direction and gradient, and to serve as water quality sampling and compliance points. Historical groundwater quality is poor. Some existing wells currently exceed AWQS for nitrate and fluoride, and total dissolved solids content ranges from 1,860 to 4,920 milligrams per liter.

Changes in groundwater levels in response to flood events in the Gila River were evaluated. Depth to groundwater in June 2002 ranged from 238 to 274 feet. The vadose zone between the designed bottom of the landfill and groundwater level ranges from 138 to 179 feet in thickness. The largest mean monthly streamflow through the Gillespie Dam gaging station, upstream of the landfill site, occurred in January 1993 and resulted in the collapse of a 135-foot section of the dam. As a result, the downstream Painted Rock Dam also reached capacity, causing floodwaters to back up along the upstream portion of the Gila River. This flooding did not impact the landfill site because the lowest elevation at the landfill is approximately 90 feet higher than the Painted Rock Dam spillway elevation. Measured increases in groundwater levels in response to the 1993 flood ranged from 40 to 70 feet in the vicinity of the landfill site as represented by data collected in November 1993.

The data indicate that the groundwater response to peak flood events imposed on post-development water levels will not impact the landfill at the design depths. In addition, improvements in flood control structures in the Gila and Salt river drainage systems, including increased capacity at Roosevelt Dam and structures in the Salt River Valley, are expected to reduce the magnitude of future peak flow events in the Gila River and groundwater level responses in the aquifer at the landfill site.

For more information, contact Greg Bushner at [email protected].

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Generalized cross section showing landfill liner construction and distance to groundwater.

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