10) Regional Groundwater flow

what is a recharge area

flow directed downward at the water table

occurs under topographic highs (hills)

generally larger than discharge areas

what is a discharge area

flow directed upward at the water table

smaller than recharge areas (usually 5-30% of watershed area)

generally higher flux than recharge areas

occur under topographic lows (valleys)

what is a hinge line

separates recharge areas from discharge areas

is precipitation counted on stream? why?

no, because its such a small area of the watershed, its insignificant, but on recharge area its much larger, so it matters

what are the 3 major influences on regional groundwater flow patterns?

topographic driving forces (water table configuration)

basin geometry (ratio of depth to lateral extent)

basin geology (variations in K from heterogeneity and anisotropy due to layering)

how does topography influence groundwater flow

water table mimics surface water boundary

undulating water table results in complex flow patterns having multiple recharge and discharge areas
local flow systems are superimposed on top of intermediate and regional flow systems

short range flow system (shaded region) is superimposed on top of a longer-range flow system

more local pathways

what are the types of flow systems and details on them

intermediate: at least one local flow system between their recharge and discharge

regional: recharge area in the basin divide and the discharge area in the valley bottom, traverse entire system

local: recharge and discharge are not interrupted by any other flow system

what is a stagnation point

where three flow systems intersect

magnitudes of the vectors in the flow field are equal but opposite in direction and cancel each other out

can also be near impermeable boundaries

how does basin geometry influence groundwater flow

ratio of groundwater basin depth to lateral extend

shallow long basin or deep sedimentary basin

in a deep basin, water moves slowly and circulates deeply

how does heterogeneity influence groundwater flow systems

groundwater flow systems typically comprised of horizontal or slightly sloping layers. different layer shave varying K values

groundwater preferentially flows along high K layers and across low K layers

shapes and patterns, layers and characteristics

how does flow in low K over high K with an increasing difference between the K values

as the difference increases, the low K zone adopts more vertical flow because more flow is carried in the lower unit

how is flow influenced with a high K layer over a low K layer

potential field is similar to isotropic aquifer, most flow is carried in upper, more conductive layer

how is flow influenced with a low K layer over a high K layer

depending on the difference in orders of magnitude, flow will flow towards the high K layer, varying degrees of vertical-ness, and then flow horizontally in the high K layer

details about low K over high K situation

represents confined aquifer

hydraulic gradient is greater in the confining bed

frictional resistance to flow is greater in confining layer, most of available energy is dissipated there

what is the prairie profile? why does saline soil occur in discharge areas there?

higher K layers at depth, where clay-rich glacial til is underlain by inter-till aquifers

due to evaporation

what are flow reversals? what causes them?

where flow is not in the same direction as the majority of the flow in the area

undulating water table

how does anisotropy influence groundwater flow

when horizontal conductivity is greater than vertical, flow paths tend to be shallower

explain this

a) homogeneous, isotropic: more evenly distributes groundwater to multiple areas

b) overlying low K: more vertical movement, the higher topographic streams will dry up, all water flows straight down and away, then converges in lower stream

c) overlying high K: may see seepage face, water table intersecting with ground

what are groundwater functions in relation to streams?

maintain baseflow during dry periods

regulate temperature of water around springs

bring nutrients into the stream

support stream-side (riparian) vegetation

higher topographic system is usually __?

recharge area

types of groundwater interactions with streams

gaining streams
losing streams
bank storage
flow through
parallel flow
coastline exchange

details on gaining streams

water table at the stream is lower than surrounding groundwater

groundwater gradients are upward

maintains baseflow

GW flow converges and equip lines point upstream

in map view, what does a V pointing upstream indicate?

gaining stream

details on losing streams

can be connected or disconnected from stream

when connected, water flows directly from stream channel into adjacent groundwater
when disconnected, WT mound below stream

GW flow diverges and equip lines point downstream

details on bank storage

as stream level rises during rainfall, gradients can be reversed causing water to flow from stream into subsurface along banks

can cause significant flooding

details on flow-through streams

WT adjacent to the stream is higher on one side of the channel where GW enters and lower on the opposite side where GW exits

equip lines are parallel to stream channel

details on zero-exchange streams

parallel flow

local water table elevation mirrors the stream stage

zero exchange

no gradient, WT contours the stream at right angles and flow is parallel to the channel

details on coastal-exchange streams

presence of brackish and seawter

GW discharges when the shoreline WT is higher than the ocean stage

ocean levels rise higher than the groundwater levels, seawater infiltrates into shallow groundwater system

flow through conditions not applicable, but zero-exchange occurs

what do location and magnitude of GW-SW exchange depend on?

natural distribution of heads, hydraulic conductivity, anisotropy distributions, boundary conditions

what does presence of anisotropic conditions cause?

directs flow preferentially to zones of higher hydraulic conductivity

what does low permeability cause in heterogeneous settings?

limited movement of exchange waters in the subsurface

details on perennial streams

permanent streams defined by water flowing all year

common with abundant rainfall, groundwater discharge, low rates of evaporation

details on ephemeral streams

don’t flow all year, only in response to precipitation

common with low annual rainfall, low water table, high rates of evaporation

which type of flow is the largest component during a storm? what is it?

overland flow

when precipitation rate exceeds infiltration capacity of the soil, occurs only during intense storms or when soil is saturated or frozen

more common in urban areas

old water vs new water

old water: been in the watershed for a while (could be soil water, groundwater, etc)

new water: water added to watershed during precipitation event

what is celerity? how is it different from velocity?

rate at which hydraulic head “wave” propagates from one pond to another

vastly greater than velocity which is how fast water moves through the aquifer

true or false: a stream can be gaining and losing at the same time

true, at different locations along the stream conditions can be different

what is depression focused recharge

hummocky (moundy), terrain creates closed drainage basins where rainfall collects in topographical depressions

is it more saline in discharge or recharge areas

discharge, and may have more salt tolerant vegetation

recharge has more flushing of water, less saline

when is depression focused recharge most likely?

in spring due to snowmelt

K in surficial sediments often ___ with depth, why?

decreases

compaction and reduced porosity
cementation and diagenesis
decrease in bioturbation and root activity
lower fracture density

how can vegetation affect seepage patterns?

transpiration causes surface water to move into GW during day and GW to move into surface water at night

what does rate of water flowing into stream depend on?

hydraulic gradient

hydraulic conductivity of stream bed and aquifer

what is hyporheic exchange? associated with? results in?

localized recharge and discharge

associated with changes in streambed topography or meandering streams

results in zone of increased chemical and microbiological activity near streams and rivers

mixing in anaerobic and aerobic

what are hyporheic zones

hotspots for biological and chemical activity that alter stream chemistry and biology

types of springs

depression, contact, joint/fracture, fault, shear/fault zone, karst

what are ways to estimate GW-SW interaction

mini-piezometer

seepage meter

water balance

hydrograph separation and tracers

models

temperature tracers

what is a seepage meter

seepage from bottom sediment collected in plastic bag to estimate q from q=Q/A

show highly variable flow from point to point

how can we use water balance to estimate GW flux?

input flux - output flux

precipitation + surface inflow + GW inflow - evaporation/transpiration - surface outflow - GW outflow

example of two groundwater flow regimes

later in summer, more stable, deeper GW flow paths

earlier in summer, responsive to fluctuations in precip and snowmelt, shallower flow paths

how can tracers be used for GW estimates

determine source areas of water and dissolved chemicals in basins

calculate hydrologic and chemical fluxes between GW and SW

calculate water ages

determine average rates of chemical reactions during transport

how do we use isotopes to determine GW-SW interactions

assume significantly different sources, the event maintains constant isotopic content

have to see vastly different isotope content for method to work

how do we use temperature methods to estimate GW interactions

fluxes into or out of a stream can be measured indirectly using subsurface temperatures

quantified based on the shift in amplitude and phase of surface T fluctuations (usually diurnal)

lakes vs streams in groundwater interactions

lakes: less rapid water level change

less surface area

bank storage less important

less shading = more evaporation

more organic deposits in sediments

groundwater discharge to lakes is generally greatest ___?

near shore

where do wetlands form? how?

areas of groundwater discharge

due to complex local and regional flow systems or along groundwater springs that form near topographical breaks

resiliency of fen vs bog

fen: recharging, more resilient to water loss

bog: not recharging, sever water table drop if that happened

types of overland flow

hortonian: rainfall exceeds infiltration capacity of soils, rainfall collected on surface moves as a thin sheet of water. controlled by infiltration rate

partial area: infiltration capacity may only be exceeded in riparian areas

saturation overland: direct water input to saturated area plus return flow contributed by groundwater coming out of land

subsurface storm flow: movement of water through unsaturated zone in downslope to stream

macropore flow: when water surrounding soil is not saturated, bypass flow

saturated wedge throughflow: impermeable layers and/or significant soil textural contrasts exist at depth, can rapidly reach stream, bypassing deeper permeable layers

groundwater ridging: small increases in soil moisture results in bump in water table (it’s the capillary fringe)

transmissivity feedback: more porous soils tend to be near surface, water table rises and rapid delivery of lateral flow

saturated areas are ___ later in the year

smaller