Model Results

All of the MODFLOW data sets are included in the electronic version of this report (Appendix A) or on attached CD of the written report.  Before testing the effects of future pumping, the current condition of the aquifer needed to be established.  A steady-state model was developed for both recharge cases to simulate conditions prior to pumping.  When using a recharge rate of 10 Mm³, hydraulic conductivities had to be set at physically unrealistic values in order to avoid large scale basin flooding.  This is further evidence that recharge at this magnitude is infeasible.  Existing well pumping was simulated beginning in 1975 to present day (2005).  Future projections where then modeled to the year 2025. 

Figure 9 illustrates the simulated present day (2005) and 2025 groundwater levels with no population growth and a recharge rate of 2.1 Mm³.  Negative elevation values indicate that the groundwater level is below that of the Sea resulting in sea water inflow. It appears from the groundwater elevation iso-lines that saline intrusion may already be occurring at the interface with the Sea of Cortez.  Without population growth there is clear evidence that saline intrusion will be significant by 2025. 

A closer examination of the annual water budget for the basin confirms saline intrusion has been occurring in the aquifer since approximately 1999.   This is corroborated by several local residences who have informally reported saline intrusion into their wells.  Figure 10 illustrates the rates of saline water intrusion compared to freshwater recharge rate.  Until 2005 existing pumping rates are used, from 2006 to 2025 pumping is increased to levels supporting the lowest proposed population growth of 30,000.  By 2025, under Plan Propuesto growth scenario, the quantity of saline water entering the aquifer is approximately equal to the amount of freshwater entering.  It is safe to assume that at this point the entire aquifer is contaminated.

All of the growth scenarios studied, even using the recharge rate of 10 Mm³, resulted in saline intrusion at the municipal wells.  The location of the municipal wells is a considerable distance from the interface with the Sea of Cortez.  This distance and the narrow constriction may slow the rate of intrusion at the well sites, but will not prevent it.  The dates of intrusion varied with projected population and plan type (Figure 11).   

Figures 12 through 15 illustrate groundwater levels in 2025 with target populations of 30000, 60000, 90000, and 120000.  Negative elevation numbers indicate groundwater levels below sea level and indicate the magnitude of saline intrusion.  The larger the 2025 population level the more groundwater decline occurs.