Water Works 20 - 4/2/03

2-Inaccurate Estimate of Irrigation Well Impacts

( Excerpts from Fellow Correspondent’s letter, continued. )

Radius of Influence of Proposed Irrigation Well.

This section (i) projects a radius of influence for this well using the same leaky-aquifer method applied above for projecting drawdown. The methodology did not work. The method appears to predict far too much drawdown at the production well, and probably underestimates the radius of intercept. The underestimate of radius of intercept is again due to the assumption of unlimited availability of recharge water from the overlying soil zone.

The NJGS then attempts to calculate a radius of influence based on a mass-balance approach. The mass-balance approach as used has numerous problems and flaws. First is the recharge value selected by the NJGS for this analysis. Second is the mapped distance to zero drawdown discussed in the last paragraph of that section on page 11 of the NJGS report. As discussed above in this letter, some of the wells the NJGS is characterizing as having no drawdown did, in fact, appear to have drawdown. Third, associating the mass-balance distance drawdown plot with the aquifer test zero drawdown line is also inappropriate, because the aquifer had not stabilized at the end of 96-hours. Additional drawdown would be expected.

As a starting point the NJGS must use the projected drawdown from the pumping well through the observation wells. This will result in a projected radius of influence of at least 9,000 feet. The NJGS should look at the field data distance drawdown plot, and then evaluate the radius of influence without trying any model that cannot accurately portray complex field conditions.