Groundwater

P = Q + ET + G + ∆S

What is an Aquifer?• Rocks and sediments have pores (spaces)

– Rock (limestone, granite, sandstone etc.)– Sand and gravel

• When pores are full of water that media is saturated• Contiguous areas of saturated media form an aquifer• Aquifers can be layered

– Confined vs. Unconfined

• Aquifers have the capacity to transmit water through interconnected pores

Floridan AquiferExtent

High Plains Aquifer

(Ogalalla)

Confinement

Cross-Section– Floridan Aquifer

Transmissivity

• How much water can be moved horizontally– Function of thickness

and Ksat

– Good measure of well productivity

– Floridan is the most transmissive aquifer in the world

Loss of Potential

Artesian Springs• Where a confining layer exists, there may be a

pressure potential in the aquifer HIGHER than the gravity potential of the surface.

• When tapped, water flows upwards

Potentiometric Surface

• Elevation of “free water surface”

• Where this surface and the ground intersect (and there’s no confinement) water seeps

Relevant Questions

• Where is the water going?– Potentiometric (piezometric) surface

• How much water is moving? How quickly?– Potentiometric surface and Darcys Law

• What level of natural assimilation is occuring?– Water quality modeling

Gainesville’s Well Field

Also Lake City

Jasper

Geology

Land use

Conservation Easement

Murphree Wellfield Cone of Depression

1988 (Observed) 2010 (Predicted)

How to make a sinkhole

Pray for lots of Rain

Suck a lot of water

Freeze Protection• To protect strawberry yield during a January

freeze in 2010, ~ 2 billion gallons per day of water pumped over a 5 day period. Voila.

Invasion

Fire

Subsidence

Degraded wildlife habitat

Soil water movement across a watershed boundary.

P=Q+ET+G+ΔS

Qgroundwater= K A ΔH/L

Darcy’s Law

Darcy’s law can calculate vertical leakage through a clay layer AND lateral flow through a seepage face.

Q is water crossing the defined area of the boundary in m3/day

K is hydraulic conductivity, or the capacity of the media to transmit water

Most meaningful as saturated hydraulic conductivity Ksat (m/day). Why?

Q= K A ΔH/L

Measuring Ksat

• Constant Head Method– Constant depth of water

(~50mm) on top of a saturated soil column with known dimensions (diameter, length)

– Outflow volume collected over a period of time (e.g., 5 hrs)

– Darcy’s Law to solve for Ksat

Measuring Ksat• Hvorslev Method

– Field measurement in screened wells

K only determined

2 4 6 8 10minutes

.1

1

.2

.3

.4

.5

.6

.7

.8

H/H o

t37

Log scale

Linear scale

H/Ho=.37

casin

g

Gravel pack

ScreenLe

Le/R must be >8

Rhigh K material

r

Approximate Ksat and Uses

Ksat (cm/h) Comments

>50 Beach sand/Golf Course Greens

5 Very sandy soils, cannot filter pollutants

0.5 Suitable for most agricultural, recreational, and urban uses

0.05 Clayey, Too slow for most uses

<0.005 Extremely slow; good if compacted material is needed

ΔH is the difference in H between two points

Water flows from high pressure to low pressure (could be “up”)

H = level of water in an open well above some datum

HA = ? HB = ? ΔH = ?

Flow Direction?!?

H @ D?

H@F?

ΔH?

Direction?1098

7

654

32

1

0

ΔH AD? Dir?

ΔH BF? Dir?

ΔH AF? Dir?

ΔH/L BF?

Q=K A ΔH/LK? A? ΔH? L?

Vertical leakage problem

Ksat=0.001m/d

Watershed=100 ha

0

1

2

3

4

5

6

Datum

Q = K * A * ΔH/L

K = 0.001 m/d

A = 100ha = 1,000,000 m2

ΔH = 5-3 = 2m

L = 2m

Q = 0.001m/d x 1,000,000m2 x 2m / 2m

Q = 1,000 m3/day or 365,000 m3/year

Q surface depth = 365,000 m3 / 1,000,000 m2= 0.365m

Q=K A ΔH/L

K=0.1m/d

A=100m x 50m=5,000m2

ΔH=108m-105m=3m

L=1,000m

Qm3/d= 0.1m/d * 5,000m2 *0.003 = 1.5m3/d

Lateral leakage problem

Groundwater Flowpaths at Streams

Groundwater Discharges

Groundwater Flowpaths

Next Time…

• Soil Water Storage