FIELD TESTING AQUIFERS TO ESTIMATE HYDRAULIC PROPERTIES (Field Follies) James Robinson, Goodwyn, Mills and Cawood, Inc.
Objectives Estimate t hydraulic coefficients; i Test conceptual models of aquifer; Facilitate numerical modeling.
Successful Tests Depend On: Correct Conceptual Model of the Flow System; Good Aquifer Test Design; Accurate and Sufficient Data; Reliable Equipment and..
A little luck!
Conceptual Model of Flow System The choice of theoretical ti model is a crucial step in the interpretation of pumping test. If the wrong model is chosen, the hydraulic characteristics ti calculated for the real aquifer will not be correct. A troublesome fact is that theoretical solutions to well-flow problems are usually not unique. Kruseman and de Ridder, 1991
Aquifer Test Design Based On: Needs of the study; Physical limitations; it ti Resources ($); Conceptual model of flow system;
Typically Assume Darcian Flow Q = -ka(dh/ds) V = K(dh/ds)/n Travel time = distance/v
Test For Darcian Flow Drawdown increases with pump rate and time; p p ; Drawdown decreases with distance from the pumped well.
Accurate and Sufficient Data?
Reliable Equipment
Good Operators Failed to convert data from Metric to English Units Properly Converted Data
Case Study Withdrawal and Tracer Tests in a Shallow Aquifer
Aquifer Test Design
Raw Data
Data Analysis - AQTESOLV
Assumptions for Analytical Solution Units tested have an infinite aerial extent; Aquifer and confining unit are homogeneous, isotropic, and uniform over the area tested; The potentiometric ti t i surface is horizontal over the area tested; Well is partially a penetrating; et Flow in the aquifer is horizontal; Flow in the confining unit is vertical; Drawdown in non-pumped unit is negligible; Ignore well bore storage.
Data Analysis Type Curves
Possible Explanations Comparison of Type curves and drawdown data suggests the conceptual model of the aquifer may not be correct; There may be a recharge boundary, the aquifer may be thicker than specified, or have a greater permeability.
Bimodal Tracer Arrival Indicates Layered Aquifer with measurable hydraulic differences
Estimation of Aquifer Porosity ALV = - k (dh/ds) n where: ALV = average linear velocity; k = hydraulic conductivity; n = effective porosity; dh/ds = the hydraulic gradient. Average linear velocity is estimated based on distance and time of arrival of peak tracer concentration. Limitation it ti non-unique solution!
First Arrival Second Arrival Porosity (%) Hydraulic Conductivity it (ft/d) Porosity (%) Hydraulic Conductivity it (ft/d) 20 20 20 11.6 25 25 25 14.5 30 30 30 17.4 35 35 35 20.3
Violates Assumptions for Analytical Solution Aquifer and confining unit are homogeneous, isotropic, and uniform over the area tested
Data Analysis Updated Type Curves
Data Analysis Numerical Model
Summary Data Analysis K T S n (ft/d) (ft 2 /d) Analytical 18 1,200 0.001 ----- Tracer 30 --------- --------- 30 1 st Arrival Tracer 14.5 --------- --------- 25 2 nd Arrival Tracer 22.2 27.5 Average Numerical 26 1,800 0.0001 ------ Model
CONCLUSIONS THE AQUIFER DOES NOT CONFORM TO THE ASSUMPTIONS UNDERLYING ANALYTICAL SOLUTIONS THE ORIGINAL CONCEPTUAL MODEL WAS INCORRECT; ESTIMATED HYDRAULIC CONDUCTIVITY AND POROSITIES ARE REASONABLY ACCURATE FOR THE LAYERS TESTED; GROUND-WATER FLOW MAY HAVE SIGNIFICANT VERTICAL COMPONENTS.
CONCLUSIONS TRANSMISSIVITY AND STORAGE COEFFICIENT ARE PROBABLY NOT ACCURATE; IN FACT, If a groundwater problem has three-dimensional overtones, it is best to revert to the use of hydraulic conductivity it and specific storage Freeze and Cherry, 1979; THERE MAY BE A RECHARGE BOUNDARY INFLUENCING DRAWDOWN DATA.
CONCLUSIONS AQUIFER TESTS PROVIDED USEFUL DATA AND INFORMATION ABOUT THE COMPLEXITY OF THE GROUND-WATER SYSTEM; PREDICTIONS ABOUT HOW THIS SYSTEM WILL RESPOND TO PUMPING MUST BE MADE CAUTIOUSLY BECAUSE OF THAT COMPLEXITY.
Another Successful Project!