COMPARISON OF CONVENTIONAL AND  THERMONIC METHODS FOR EVALUATING GROUND-WATER BUDGETS - SANTA FE COUNTY, NEW MEXICO, U.S.A.   

William M. Turner, Ph.D. 
 

INTRODUCTION   

The Ninemile Ranch area is situated about nine miles (14 km) southeast of Santa Fe, New Mexico.  The area is the site of a small water-well field which supplies residential users in the immediate vicinity.  The anticipated growth in population of the area created a concern over the ability of the well field to meet the needs of future residents. 

AGW scientists conducted a detailed hydrogeological study of the area to determine the amount of annual average recharge to the aquifer exploited by the well field.  Conventional hydrogeologic and our proprietary Thermonic methods were employed in the study.  This case study report demonstrates the value of Thermonic methods in achieving more accurate ground-water budgets.  Information determined by Thermonic methods serves also as an independent check on information determined by other methods.

HYDROGEOLOGIC SETTING   

The Ninemile Ranch area is situated within the Santa Fe embayment of the Espa?ola Basin which occupies the Rio Grande structural depression of central New Mexico.  The area is comprised of Precambrian granite to the east and Quaternary to Tertiary continental deposits to the west. 

The sedimentary deposits wedge out to the east over the granitic basement.  The primary aquifer is the Galisteo Formation.  The Galisteo Formation is comprised of interbedded sand and red-shale beds.  The Galisteo Formation dips about seven degrees to the west.  Throughout most of the well field, ground water is under artesian conditions. Ground water in the granite to the east is under water table conditions. 

Recharge to the aquifer system occurs along the mountain front and by infiltration through the beds of the major ephemeral stream channels.  Very little recharge occurs through direct precipitation on the land surface.  Ground-water inflow from the granitic terrane east of the well field contributes to recharge of the aquifer.  Average annual precipitation in the area is 13.78 inches (350 mm).  The climate is semi-arid.  A net annual water deficit exists at the land surface. 

The potentiometric surface in the area of the well field shows a cone of depression centered about Well 1.  Wells 1 and 2 were in operation for about 6 years prior to the study.  Well 2 had less total production than Well 1 and was shut down earlier than Well 1.

CHOICE OF METHODS  

The aquifer system and its boundaries are well defined.  Conventional hydrogeologic methods were employed in the study.  Hydrogeological methods are based on data from an aquifer that is actually exploited.  Estimates of ground-water recharge to the aquifer were refined considerably by the application of Thermonic methods. 

Recharge to the Ninemile Ranch Area is the sum of mountain-front recharge, stream-channel recharge, and ground-water inflow from the granitic terrane to the east.  Under non-pumping conditions, all of this water eventually leaves the area of the well field as ground-water outflow.  The total annual recharge equals the annual ground-water outflow plus artificial abstraction plus or minus the change in ground water storage in the area.  In the present case, we assumed the change in aquifer storage over the period of one water year caused by well production or change in recharge would be negligible.  Therefore, the amount of ground-water outflow is equal to the ground-water recharge and can be determined from Darcy's Law and flow-net analysis.

HYDROGEOLOGICAL ANALYSIS  

For the flow-net analysis of the ground-water-flow system in the Ninemile Ranch Area, AGW scientists subdivided the ground-water-outflow section into several subsections.  Transmissivity values were determined from long-term aquifer-performance tests (49 - 96 hours) and assigned to each outflow subsection.  The outflow subsections are bounded by ground-water-flow lines.  The ground-water outflow across each of the subsections was determined by Darcy's Law using the measured values of hydraulic gradient and calculated values of transmissivity. 

Outflows across the subsections were summed to determine the outflow across the total outflow section. Standard hydrologic methods give total ground-water outflow of 284 acre-feet per year.

THERMONIC ANALYSIS  

Convention assumes the ground-water-flow lines are normal to the equipotential lines.  Othogonality exists only for material which is isotropic for fluid flow.  The aquifer under study is not isotropic for fluid flow.  Because of limited data, Othogonality was assumed. 

Using conventional hydrogeologic methods, the total outflow section was subdivided into a number of subsections. because the aquifer is not homogeneous and the transmissivity varies throughout the aquifer.  Still, the outflow subsections were subjectively determined.  For greater accuracy, it is necessary to determine the hydraulic gradient and the transmissivity at many points along the outflow section by drilling wells and conducting aquifer-performance tests.  This is not practical. 

Thermonic methods can be used to rapidly and inexpensively supplement transmissivity determined from aquifer-performance tests.  Thermonic methods permits the outflow section to be more reliably subdivided into subsections each having a characteristic value of aquifer transmissivity. 

In the present example, where aquifer-performance tests were conducted, values of transmissivity were assigned directly to parts of the outflow section.  Transmissivity values obtained both by Thermonic and aquifer-testing methods were in agreement with each other.  Where values of transmissivity were not available from standard methods, we calculated or estimated them by Thermonic methods. 

Where the aquifer area sampled by an aquifer-performance test is small, Thermonics can be used to add to and extend the aquifer sample size.  Thermonics can determine how representative an aquifer-performance test site is in relation to the entire aquifer.   AGW scientists used Thermonic methods to re-subdivide the ground-water-outflow section into more subsections.   Total ground-water outflow using conventional methods augmented by Thermonic methods give a ground-water outflow of 167 acre-feet per year (205,550 m²/y).   

OUTFLOW SECTION	WIDTH (ft)	TRANSMISSIVITY (gpd/ft)	GRADIENT	OUTFLOW   (af/yr.)
L1	592	125	0.035	2.90
L2	1057	1000	0.026	30.80
L3	815	800	0.020	14.62
L4	98	1200	0.018	2.37
L5	1086	300	0.016	5.84
L6	195	6500	0.014	19.89
L7	544	2000	0.013	15.86
L8	553	10000	0.012	75.39
			TOTAL	166.67

CONCLUSION  

We believe the lower amount of ground-water outflow from the Ninemile Ranch area is more reasonable.  Our conclusion is based on estimates of the ground-water yield from precipitation in the Ninemile Ranch area and in the granitic terrane to the east.  Our conclusion is consistent with estimates of the ground-water outflow of adjacent and nearby areas made by other workers using different methods of analysis. 

We conclude that Thermonics is able to significantly improve recharge determination made by conventional hydrogeologic methods. 

The method is rapid and inexpensive.  It has the added advantage of identifying zones where higher capacity water wells could be drilled.

? 1999 AGW Consultants.  All Rights Reserved.