Date of Award

1994

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

P.J. Gerla

Abstract

Many communities in North Dakota rely on water from shallow, unconfined aquifers that are vulnerable to contamination. A strategy to protect public groundwater supplies is the delineation of wellhead protection areas (WHPAs) around the area which contributes water to a well. Several delineation methods are available, all of which have simplifying assumptions which affect the size or shape of a predicted WHPA.

Five WHPA delineation methods were compared to determine which simplified the aquifer/well system while preserving essential hydrogeologic features. A simple system may not require a complex model to predict an accurate WHPA. Sensitivity of each to changes in values of hydraulic conductivity (K), specific yield, porosity, and recharge was also evaluated.

The study site was the municipal wellfield at Larimore, North Dakota. Three wells tap the shallow, unconfined Elk Valley aquifer. The water table is 1 to 25 feet below the surface and the base of the fine to medium sand aquifer is 7 to 62 feet deep.

Arbitrary (AFR) and calculated fixed-radius (CFR) methods produce circular WHPAs. The AFR does not use site-specific data, and the CFR varies only with porosity.

The semi-analytical models, RESSQC and GPTRAC, generate WHPAs using simplified flow fields based on aquifer and well data and particle tracking. The shape and position of RESSQC WHPAs were sensitive to changes in K. GPTRAC WHPAs varied with changes in K, porosity, and recharge.

The MODFLOW/SURFER/GWPATH (MSG) method combines a numerical groundwater flow model (M), a contouring program (S), and a particle-tracking program (G). The hydraulic head distribution from a cone of depression generated by MODFLOW is combined with digital map of known heads for use as input to GWPATH. The size, shape, and location of MSG WHPAs varied with K and specific yield. Recharge is implicit in the hydraulic head map.

MSG most closely represented the aquifer/well system and is assumed to produce the most accurate WHPAs. Neither fixed radius method was realistic for the aquifer at Larimore. RESSQC and GPTRAC, without recharge generated WHPAs that were too large. GPTRAC with recharge generated WHPAs most similar to those produced by MSG and is probably adequate for the simple system at Larimore.

The Larimore wellfield and aquifer are typical of many towns that depend on water from shallow aquifers. These delineation methods should apply to similar localities.

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