McKownville
Improvement Association
McKownville groundwater in the Colonie Sand
Groundwater in McKownville
The Colonie Sand, on which essentially
all of McKownville is built is, like all unconsolidated sands, a
porus material with about 25-30% void space between the sand grains.
These pores in the material are connected around the sand grains,
and so any fluid in the pores is able to flow in response to a
pressure gradient; such material is said to be permeable to fluid
flow.
Beneath the surface at most times, the pores in the upper few feet
of the sand contain mostly air but, at some depth, typically 6 to 9
ft or so below the surface in McKownville, the pores are filled with
water. The water is there because much of the rain and snow melt
percolates down through the pores in the sand and fills the sand up
to an elevation where at any time a balance is reached between the
amount infiltrating from the surface and the amount flowing slowly
away, through the sand, to local streams. This top of the filled, or
saturated zone is called the water table (or groundwater table).
Because the rate of supply from precipitation and infiltration
rarely exactly equals the rate of outflow to local streams, the
level of the water table changes with time, rising when the
infiltration is more than the outflow, and falling when the outflow
exceeds the local infiltration. In many places in temperate climate
regions near-surface aquifers (geological layers in which
groundwater flows) show a characteristic rise in the water table in
winter months and a fall through the summer months, mainly because
more of the rain evaporates, and is transpired from the soil by
trees and other vegetation, and so less replenishes the aquifer
during the summer interval. See this in the monitoring
well record below.
The flow in
streams, like the Krum Kill, is at most times fed from the
groundwater flow in the local near-surface aquifer; (other than
brief intervals in and a few days after heavy rain storms or snow
melt events where there is direct run-off of water from surfaces
into the streams, or from storm drains fed to them). Flow times for
a sample drop of water infiltrating near the upper end of recharge
areas in such aquifers are typically several years to travel to the
local stream.
The term water table is potentially a bit misleading, because it is
not a flat surface, but (if not disturbed by pumping from wells)
tends to follow and be a subdued copy of the local ground surface
topography, as show in the diagram on the left. These differences in
elevation of the water table create a natural pressure gradient
which is the cause of the flow in the aquifer. The water table
intersects the surface of running streams, and ponds and lakes if
they are not lined with impervious (or impermeable) material, such
as plastic sheet, or clay. It also intersects the ground surface at
natural springs, which are commonly at the heads of streams in the
area of the Colonie Sand.
Krumkill west branch stream at McKown Road, 26 Feb 2018. Some modest
quantity of precipitation (rain) had occurred in the week prior to
this date, but non-flood flow in the stream as shown mostly comes
from groundwater discharge out of the Colonie Sand, not from runoff.
for larger image, click here
or on the image
Geological profile section for groundwater in the Colonie Sand
aquifer in McKownville. The vertical scale is enlarged relative to
horizontal scale in order to show clearly the water table and its
relation to the ground surface.
Location shown on map below
In McKownville (and in most of the larger area covered by the
Colonie Sand), the water infiltrating the ground is unable to
percolate to significant depths, because the geological layer of
clay-rich material, the Albany Clay, underlies the Colonie Sand, and
forms the base to the aquifer. [more
information on the geology of McKownville]
The properties of clay, with very fine clay mineral particles packed
closely together, do not allow fluids to flow through them, or only
at such slow rates under natural pressure gradients that they are
essentially a barrier to flow of water, or most other fluids. So
water that enters the Colonie Sand stays in the sand layer, above
the Albany Clay, flowing out only to local springs and stream
channels at or above the elevation of the base of the Colonie Sand.
This borehole-based record (from the University at Albany campus,
near Indian Pond) shows the elevations of the upper and lower
surfaces of the Albany Clay as they are in McKownville. The upper
surface of the Clay, the base of the Colonie Sand, is known (Dineen,
1982) generally to rise going north and west from here, being
located up to or a bit higher than 250 ft above sea level in the
area west of Route 155 near the NY Thruway. Flow patterns of
groundwater in the Sand are probably affected by this significant
change in elevation of its base. The top of the Sand in that area
(the ground surface), is about 320 ft elevation outside the stream
valleys, again with dunes up to 50 ft high on top.
USGS water monitoring well A-654 located on
the University at Albany campus. Water depth in the well is measured
automatically every 15 minutes and the data transmitted via the
satellite antenna on the pole.
Graph shows the depth to the water table measured in this well
between November 2018 and July 2020; the decline seen during the
summer months (May-October) and the recovery back up during the
winter is a fairly consistent pattern for this well in the Colonie
Sand aquifer.
Data, graph, photo and well description from USGS website.
location shown on map below
Views of groundwater in McKownville
Infiltration pond in construction next to Dutch Quad, SUNY Albany
(no lining, this shows the water level (~245ft asl) of the water
table in the Colonie Sand here on 5 December 2018)
location P on map below
click here or on
the image for larger version
excavation on Elmwood St to install new water main showing effluent
from pump in the excavation, and the source: saturated sand of the
top of the water table ~ 5ft depth (~221 ft asl) in the
excavation, 29 November 2018
location E on map below
click here, or here, or on images for
larger version
Spring of water from the Colonie Sand in the lot of 1257 Western
Avenue, 19 March 2018. The darker and iron-stained area, indicated
with the red arrow, is damp from the seepage, which is controlled
here by the top of the Albany Clay at this elevation, 190 ft asl; location S on map below.
click here or
on the picture for larger version
The house formerly
in this lot was plagued by water infiltration into its
basement, and rapidly deteriorated after it passed in 1973 from
owner occupancy to non-resident property; it was demolished in 2004.
Later in the year this picture was taken, this lot was levelled and
partly filled with gravel to support construction activities for new
water mains and storm drains in nearby streets of McKownville. This
fill has disguised the high water table here, but does nothing to
remove the problem for future uses of the property.
Seepage and events of flooding of groundwater into basements of
McKownville houses are a direct consequence of the often high level
of the water table in the Colonie Sand. The Town Of Guilderland has
a construction project funded and in
progress since 2018 to replace old water main pipes and
install a proper storm drain system in the area of McKownville north
of Western Avenue in order to alleviate this situation. Completion
of the project is scheduled to occur in 2021.
Map segment from USGS 7½' Albany quad,
1953 base, purple additions to 1978.
Location of groundwater profile (above) -
red line,
H- Hampton Inn
E- Elmwood St water line excavation photos
P - infiltration pond at Dutch quad photo
S - spring in 1257 Western Ave lot photo
The Colonie Sand contains in some places, far above its base on the
Albany Clay, a thin (3-6 ft) clay layer which occurs at about 300 ft
asl elevation in areas west of Rte 155 and, according to Dineen
(1982), this persists to the east at least as far as the Adirondack
Northway near its intersection with I-90. The elevation of this
layer in the sand declines to the east down to about 250 ft asl.
This layer appears to extend just into the northernmost part of
McKownville west of Fuller Road, where it is probably responsible
for the wetland north of Warren St and western Mercer St, whose
ground surface is at 255 ft asl. Although thin, this clay layer is
an aquiclude, sufficiently impermeable so that groundwater is ponded
above it, a situation termed a "perched" water table, illustrated in
the diagram below. The diversion of runoff from the SUNY Polytechnic
Insitiute to a detention pond in this swampy area has likely raised
the water table elevation in this part of McKownville and made more
difficult the remediation of drainage here. Sending this runoff into
the Patroon Creek groundwater and drainage area, where originally it
would have gone if not diverted, would be helpful.
Detention pond in the eastern end of the swampy area north of Warren
and western Mercer Streets, McKownville; houses on Warren St seen
beyond the trees in the center. The pond receives runoff from SUNY
Albany Freedom Quad and much more from the large built-over area of
SUNY Poly. This swampy area in which this retention pond was placed
is here because of a perched water table over the clay layer within
the upper part of the lake sands of the Colonie Sand.
click here or on the
image for a larger version
The origin of this persistent varved lake clay layer up in the
Colonie sand is an intriguing geological question. Dineen (1982)
thought that it was from deposition in a lagoon behind a sand
barrier bar in Lake Albany, which would require it to be younger to
the east, as the lake shore and the barrier bar migrated in that
direction while the lake level was dropping slowly. Alternatively,
if it is synchronous across the area, which its continuity as a
single layer tends to suggest, an event temporarily interrupting the
sand supply from the Iro-Mohawk
river is implied, which could be a short-lived re-advance of
the Laurentian ice front that for a brief interval of time dammed
off the drainage down the Iro-Mohawk.
Dineen, R.L., 1982. The geology of the Pine Bush Aquifer,
north-central Albany County, New York. New York State Museum
Bulletin 449, iv +26pp.
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