suitability report finalgrpone_enviroplan
TRANSCRIPT
Suitability AnalysisProsser Hill , Cheney Washington
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Suitability Analysis
Prosser Hill, Cheney Washington
1.0 Introduction
As requested, our environmental planning firm has conducted a thorough analysis on the
suitability of septic tank systems, agricultural capabilities, and hydrological suitability, in
Prosser Hill to determine if it is suitable for residential development. Due to the budgetary
constraints associated with this planning assignment, we were not able to go into the field.
However, our firm has used the best available data from numerous resource documents and
maps provided by the United States Department of Agriculture, the Environmental Protection
Agency and others, to analyse information regarding soil types, permeability, and land
capability classifications. We further evaluated depth to bedrock and depth to water table. All
of these components are essential to septic tank system effectiveness and are critical to the
viability of a residential development. When located in the right areas, with proper soils
and permeability, septic tanks can be very effective at treating domestic waste. Conducting
a thorough analysis to ensure proper placement and suitability is extremely important, as
improper placement can lead to property damage, ground and surface water pollution, and
disease.
In this report, you will find an analysis of the Prosser Hill suitability for septic tank systems,
agricultural capability, and detailed hydrologic information to further understand the
suitability of the area. In addition to the report, eight maps have been produced to display the
findings produced through our analysis and research.
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2.0 Suitability for Septic Tank Systems
Septic tank systems are comprised of three elements: the septic tank, the drain field, and the
surrounding soil. The septic tank is a reservoir buried beneath the surface and works to hold
and process the wastewater from homes. Heavy solids typically settle to the bottom of the
tanks and the natural bacteria decompose this material. Over time, this material will
accumulate and the tank will need to be pumped out to prevent overflow into the drainage
field. The drain field also plays a critical role in the septic tank system. A network of pipes
are buried in trenches within the soil, where the wastewater slowly flows out of the pipes into
gravel and the soil. Then the soil below the drain field serves the purpose of conducting the
final process on the effluent.
When wastewater passes through the right types of soils at the appropriate speeds, the soil is
essentially working as a filter using natural chemical and biological processes to treat the
waste. For these processes to work most efficiently and effectively, the soil needs to be
mostly dry, permeable, and have plenty of oxygen. Not only can soil work as a filter, it
absorbs organic and inorganic materials and even pathogens. Through combined chemical
and biochemical processes it produces water that is of high enough quality to be released into
the groundwater.
For this process to be most effective the soil needs to be permeable. The reason this is critical
is due to the fact that more permeable or unsaturated soil allows for the wastewater to travel
and flow through smaller holes or channels and therefore experience a higher-degree of
filtration. In more saturated soil the wastewater is routed through larger holes or pores and
does not receive a thorough filtration process.
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Soil particles can absorb bacteria, viruses, ammonium, nitrogen, and phosphorous, all of
which pose serious public health risks if not treated properly. Within permeable soil these
particles are retained long enough for the natural processes present in soil microorganisms to
prey on them until they are eliminated.
According to the research on this subject, it has been found that two feet to four feet of
unsaturated soil provides enough depth for the proper removal of harmful bacteria,
viruses, and phosphorous. However, the depth of the soil may need to be more than the
aforementioned depth if there is limited permeability present in the soil. Again, the right soil
type and saturation and oxygen levels are crucial to the effectiveness of a septic tank system.
2.1 Permeability
Permeability is the measure of the amount of water that will pass through a soil sample per
minute or per hour (Marsh, 1983). Knowing a soil’s permeability is critical to understand the
rate at which wastewater will be received and diffused into a given soil type. Permeability
plays a key role in whether or not a septic tank system in a given area will be a success or
failure. If a soil has low permeability, such as those that are saturated or made up of compact
clay, water is rejected or blocked and causes septic tank systems to back up, overflow and
even break out of the surface of the ground. On the other hand, if permeability is too high,
wastewater moves down through the soil too quickly for nutrients and pathogens to be
removed and is at risk of polluting nearby water tables and bodies of water. Taking this
information into account, the Environmental Protection Agency (EPA) has adopted standards
that describe the limitations soil permeability imposes on the suitability of septic tank
systems in a given area. These standards describe three categories in which different soil
types can fall under: Slight limitations, moderate limitations, and severe limitations. Slight
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limitations are defined by soils that have a permeability of 2.0 – 6.0 in/h. This is the ideal
range for the installation of septic tank systems, as it is not too fast and not too slow, which
allows wastewater to be treated effectively without polluting groundwater or breaking out of
the surface. Permeability rates falling in the range of 0.6 – 2.0 in/h are said to impose
moderate limitations. Lastly, permeability rates falling below 0.6 in/h or above 6.0 in/h are
said to impose severe limitations, as the rate at which the water is absorbed through the soil is
said to be too slow or too fast, respectively. The depth of the drain field outlet for septic tank
systems are specified to be at a minimum depth of 24 inches below the finished grade.
Therefore, soil permeability at a depth of 24 inches and greater are significant to the success
or failure of the system.
2.2 Permeability Methods
In the analysis of the soil permeability of Prosser Hill, the Washington State Soil Survey was
used to determine soil horizon permeability ranges within the study area. However, the soil
survey uses different permeability ranges than what the EPA uses as its standards for slight,
moderate, and severe limitations for septic tank systems. Therefore, the soil
survey permeability rates must be adjusted to reflect the EPA standards. The method for
deciding which EPA limitation each soil horizon best fit was to determined by how much of
the permeability range fell within slight, moderate, and severe (given as a percent), and to use
whichever EPA limitation was dominant in that soil horizon. To determine the EPA
limitation for the entire soil type, the amount of soil depth (in inches) for each EPA limitation
type was totalled. Any depth less than 24 inches was omitted in the calculation. Whichever
EPA limitation accounted for the most soil depth was said to be the EPA limitation for the
entire soil type. For example, for the soil type CgB:
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Depth Permeability
0-28” 0.8-2.5in/h
28-35” 5.0-10.0in/h
35-60” >10.0in/h
The first soil horizon (0-28”) had a permeability range that fell within both slight and
moderate limitations by the EPA standards. Here we found that 0.8-2.0in/h falls under EPA
moderate (0.6-2.0in/h) which accounts for approximately 70.6% of the first soil horizon’s
permeability range. 2.0-2.5in/h falls under EPA slight (2.0-6.0in/h), which accounts for
approximately 29.4% of the first soil horizon’s permeability range. Therefore, since more of
the first soil horizon falls under EPA moderate limitations, we classified the entire first
horizon as having moderate limitations. However, since we are only interested in the soil
depth greater than 24”, we said that 4” of the first horizon has moderate limitations. The
second horizon (28-35”) had a permeability range that fell within both slight and severe
limitations by the EPA standards Here we found that 5.0-6.0in/h fells under EPA slight (2.0-
6.0in/h), which accounted for 20.0% of the total permeability range for the second horizon.
The remainder of the second soil horizon had a permeability range of 6.0-10.0in/h, which
falls under EPA severe (>6.0in/h), which accounted for 80% of the second horizon’s total
permeability range. Therefore, since a majority of the second soil horizon fell under the EPA
severe limitations, we classified the entire 7” of the second horizon as having severe
limitations. The same method as stated above was applied for the third soil horizon (35-60”),
which resulted in 25” of severe limitations. The overall limitation of the entire soil
type, CgB was determined based on the ratio of slight, moderate, and severe limitations in all
the horizons. Out of the 37" of soil depth below 24" for the soil
type, CgB, 33” (89.2%) imposed severe limitations and only 4” (10.8%) imposed moderate
limitations. Therefore, the soil type, CgB was classified as having severe limitations overall.
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This methodology was applied for all soil types within the study area. Soil types were then
color-coded on the permeability map based on their limitation, with red denoting severe
limitations, yellow denoting moderate limitations, and white denoting slight limitations.
2.3 Permeability Findings
We found that approximately two-thirds of the soil within the Prosser Hill study area imposed
severe limitations to septic tank systems in regards to soil permeability (See Permeability
Map). This was most prominent in the northeastern section near Queen Lucas Lake, the
south central section southwest of Fish Lake, as well as the western section bordering the
Northern Pacific rail line. Small pockets of land in these areas imposed moderate
limitations. The remaining area of the land in between the northeastern and
southwestern sections were a mix of soils imposing both slight and moderate limitations for
septic tank systems Overall, Prosser Hill is generally unsuitable for septic tank systems in
terms of soil permeability with less than one third of the total land area containing soils that
impose slight limitations. It should be noted that the amount of acreage showing slight
limitations on the permeability map is misleading, as all the soil types exhibiting slight
limitations have insufficient depth to bedrock (< 48”), and therefore may not be suitable for
septic tank systems when this is taken into account.
3.0 Depth to Bedrock
The depth to bedrock analysis determines if a site is suitable for septic tank systems for
development. The waste created, which the septic system is designed to treat, contains
nitrogen phosphorus, ammonia and harmful pathogens like viruses and bacteria. These
elements can cause nutrient loading in the soil. The viruses and bacteria can be translocated
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into ground water and aquifers. If translocated into an aquifer or ground water system those
viruses can spread into surface hydrology systems and fluvial systems. Translocation occurs
as the untreated wastewater comes in contact with the bedrock and depending on
permeability of the bedrock, flows in the direction gravity pulls it.
The underlying bedrock of the site is mainly basalt. Basalt is considered impermeable. This
creates a problem when dealing with wastewater treatment and can transport the waste from
one area to another if the depth of soil is not deep enough to filter out the ammonia, nitrogen,
phosphorus, viruses and bacteria. How the suitability analysis determined depth to the basalt
bedrock was through the soil survey and the associated soil types of the area. The soil survey
lists depth to bedrock and/or where the soil horizons end.
3.1 Depth to Bedrock Methods
The EPA regulations state that the depth to bedrock for slight limitations are anything greater
than 72”under the surface. For the moderate limitations the depth to bedrock must be
anything under 72” up to 48”. The severe limitations for depth to bedrock is less than 48”
under the surface. Using the Soil Survey of Spokane County, Washington we color-coded
soil types with moderate limitations yellow and severe limitations red. For example the
broadax silt loam (BpB), present on the west side of Prosser hill, had a maximum soil depth
of 60” and was not deep enough for slight limitation but adequate for moderate and was
colored yellow.
The majority of our map falls under moderate limitations (yellow). Most of the severe
limitations (red) are on the northeast side of Prosser Hill or just north of Fish Lake. No slight
limitations were found within the suitability analysis area.
4.0 Depth to Water Table
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Depth to water table is an important aspect to look at while installing septic tank systems. It is
necessary to know at what depth the water table begins, so the septic system can be installed
at the right depth. There is a risk of polluting ground water when the soil is too saturated,
because it lacks the ability to filter the waste and toxins throughout the layers of the soil. This
is caused by a depth to water table that is too high. Although a shallow water table also poses
risks to surface hydrology, because it can become contaminated by surface activities
including farm animals or oil run off.
Furthermore, if the water table is too close to the drain field there might not be enough
distance for the soil to filter waste and could degrade nutrients. The shallowest depth to water
table should be used in order for the septic system to function year round. Septic tank systems
should only be considered where the limitations are considered slight or moderate. According
to the Soil Survey of Spokane County, Washington, slight conditions are defined as being
greater than 72 inches, whereas moderate is from 48 to 72 inches, and severe is less than 48
inches.
Although there are very few areas that consist of bedrock surfaces and some areas that
contain wetland marsh; both are considered to indicate severe limitations. On our depth to
water table map we used the color red for severe, yellow for moderate, and slight was left
white. Also if there was no description listed in the Soil Survey of the water table we
considered it outside our area of concern and listed it as slight. Areas of severe limitation on
our map mostly appear around the edges of the study area, with a small cluster of severe areas
in the northern part of the map.
4.1 Depth to Water Table Findings
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In conclusion, most of the depth to water table map displays slight limitations, based on this
information alone one could make the case for installing septic tank systems in most areas.
However, it is always important to consider all of the limitations and it is our
recommendation that the analysis from all of the maps and research be used to assess the
suitability of septic tank systems for Prosser Hill.
5.0 Slope
A sloped surface has a higher elevation at a specific point than it has at another. The measure
of the slope is determined by dividing the change in elevation between the points by the
distance between the points (commonly referred to as rise over run), and is then multiplied by
100 to be expressed as a percent slope1. Slope is an important factor to consider when
determining the suitability of an area for a SAS. When the slope is steep the waste being
processed collects at the lower elevation of the slope1. The rapid flow downward disrupts
natural filtration as wastewater moves too quickly to properly infiltrate the lower layers of
soil. Additionally, when the wastewater amasses in the lower area, the soil becomes too
saturated to effectively filter waste. To avoid these potential hazards, the Environmental
Protection Agency has set forth the following standards regarding the limitations percent
slope presents to the suitability of a SAS. Slopes ranging from 0-8% have been determined to
have only slight limitations2. These percent slopes allow wastewater to move slowly enough
to infiltrate through the layers of soil as needed for proper filtration. Slopes ranging from 8-
15 % have been determined to have moderate limitations2. These percent slopes require
careful consideration and planning if a SAS is to function properly. Slopes greater than 15 %
have been determined to have severe limitations2. These percent slopes should not be
considered suitable for a SAS.
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5.1 Slope Methods
In the analysis of the percent slope of Prosser Hill, the USGS 7.5 Minute Four Lakes
Quadrangle topographical map with a scale of 1:12,000 enlarged from 1:24,000 was used to
determine areas of slight, moderate, and severe limitations within the study
area. Topographical maps display changes in elevation using contour lines. The spaces
between the contours lines, known as contour intervals, represent a set change in elevation. In
simple terms, an area with minimal slope is represented by contour lines that are far apart as a
change in elevation is very gradual over the distance between two points. Steeper areas are
represented by contour lines that are close together, as the elevation changes quickly over
short distances. On the map used in this study each contour interval represents 20’ on the
actual landscape.
To determine the slopes within the study area the distance between contour lines was
measured. As the scale of the map allows only limited evaluation of slope we used a general
rule that each area being investigated was required to have 3 contour lines resulting in 2
intervals, and to be a 1/2” minimum in length. Using the rise over run formula a slope of 8 %
was determined to be one that rises 80’ over a 1000’ run. A slope of 15% was likewise
determined to be one that rises 150’ over a 1000’ run (see Fig. 1). Further, it was determined
that a distance of 1/4” between contour lines represents an 8% slope as 1/4” represents 250’
on the actual landscape, and the 20’ rise divided by the 250’ run is .08, which when
multiplied by 100 is our 8 % slope. Likewise, a distance of 1/8” between contour lines
represents an approximate 15 % slope (see Fig. 2). A measuring device was created with 1/4”
and 1/8” markings. This device was run along each contour line perpendicularly to evaluate
the distance between the nearest parallel contour line. Distances of ≤ 1/8” were determined
to represent areas of severe limitations. Distances of 1/8 – 1/4” were determined to represent
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areas of moderate limitations. Distances of > 1/4” were determined to represent areas of
slight limitations.
80'1000 '
=.08 ×100=8 %150 '
1000 '=.15 ×100=15 %
Fig. 1
20 '250 '
=.08 ×100=8 %20 '
125 '=.16 ×100=16 %
Fig. 2
Using this method it was found that approximately one-third of the Prosser Hill study area
imposed severe limitations (see Slope Map). The areas are found primarily in the center of
the study area where Prosser Hill is located, and along the eastern border where steep rock
outcroppings border Queen Lucas Lake to the north, and extend to the south towards Fish
Lake. Near the base of Prosser Hill, and in some small pockets to the north and east, there
were areas found that impose moderate limitations. Overall, it appears that greater than 50
percent of Prosser Hill is suitable for septic systems based on the slope standards set forth by
the Environmental Protection Agency.
6.0 SAS Composite
The Soil Absorption System (SAS) composite map, which is the basis for the suitability
analysis of Prosser Hill shows some areas that are suitable for development. To create this
map, all of the severe, moderate, and slight limitations of soil permeability, depth to bedrock,
depth to water table, and slope were merged into a single SAS Composite Map.
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This SAS Composite Map was created based off of the severity of three categories for the
specified map classification. These three categories were: slight limitations which was left as
a white field, moderate limitations, which was indicated by yellow, and severe limitations,
which were highlighted in red. Each map was created separately and limitations were marked
for each category. The color was transferred from each of the four base maps by hand,
starting with the most severe limitations to form new polygons on the SAS Composite Map.
After transferring all of the red color for severe limitations, the process was duplicated for
moderate limitations in yellow. Because the depth to bedrock map left no areas with slight
limitations, the SAS Composite Map only includes severe and moderate limitations that are
highlighted in red and yellow. There are no slight limitations on this map.
The combination of these maps shows pockets of acreage in the West Central and North West
Central areas of the Prosser Hill area with moderate suitability for the use of soil absorption
systems. There are also some other small sections of acreage that are located at the top of
Prosser Hill and along a few of its slopes.
7.0 Agricultural Capability Information
Agricultural capability of land is based on a cumulative survey of soils, slope, erosion and
other features1. Based on the findings in these surveys land is classified into one of two
divisions, suitable for cultivation, and not suitable for cultivation4. Land in both divisions
may have other uses such as pasture, woodland, or wildlife habitat4. In the USDA Soil
Conservation Service “Land Capability Classification” these divisions are further divided into
classes I-VIII. Classes I-IV are within the suitable for cultivation division, while classes V-
VIII are within the not suitable for cultivation division.
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7.1 Agricultural Capability Methods
There are currently no recognized state or federal regulations for determining limitations for
the development of land based on that land’s agricultural capability. Instead, suitability for
development is based on the likelihood of opposition from conservation groups. As lands that
are suitable for cultivation of crops and pasture for livestock are considered valuable natural
resources, it is presumed that conservation groups will argue against the development of these
lands for non-agricultural uses. We evaluated each class carefully to determine which would
receive severe opposition, and therefore represent severe limitations, which would receive
moderation opposition, and therefore represent moderate limitations, and those that would
receive little to no opposition and therefore represent slight opposition. We concluded that
classes I-IV represent severe limitations, classes V and VI represent moderate limitations, and
classes VII and VII represent slight limitations. As classes I-IV are suitable for cultivation
and pasture we believe conservation groups will severely oppose the loss of these lands to
development. Classes V and VI are not suitable for cultivation, but are still suitable for
pasture and therefore we believe conservation groups would question the loss of these lands
with moderate opposition. Classes VII and VII are not suitable for cultivation or pasture, and
are useful only for grazing. Therefore we believe conservation groups would not likely
oppose development in these areas.
Using this method it was found that approximately one-half of the Prosser Hill study area
imposed severe limitations (see Agricultural Capabilities Map). The areas are found primarily
on the western border and into the center of the study area, with pockets in areas further east.
Land with moderate limitations is found in a band along the eastern border of the area of
severe limitations. Overall, it appears that less than 50 percent of Prosser Hill is suitable for
development based on the agricultural capabilities of the land, and the probability of
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opposition from conservation groups.
9.0 Surface Hydrology
Bodies of water are amongst the most productive ecosystems on the planet (Marsh, 448), with
riparian zones and wetlands being diverse habitats for many species. These areas are
considered critical areas for conservation in Washington State; therefore, there are
recommendations in place to provide buffers around these zones for the purpose of protecting
them. These buffers can range in size. For example, Pierce County’s recommendations for
buffers around wetlands vary from 25 feet to 300 feet, dependent on the intensity of the
development and the wetland classification. For the purposes of this analysis, we will be
recommending 300 foot buffers around all bodies of water in the Prosser Hill study area. The
reason for this is two-fold: (1) 300 feet is approximately 1/3” at the 1:12000 scale that we are
working with. Anything smaller than this will be too small on the map to significantly stand
out. (2) There is good evidence that buffers of less than 300 feet fail to achieve habitat and
microclimate benefits that they are intended to provide (Trohimovich, T. pg. 4). We also do
not want to place septic tank systems in wetlands or around streams, because these areas
contain saturated soil that is unsuitable for absorption and filtration and because placing them
near bodies of water can lead to pollution and nutrient overloading. We know that there are
many adverse impacts of development near streams, rivers, and wetlands, including:
increased water temperatures, the loss of organic debris, decrease in water quality, and
changes in microclimate adverse to fish and wildlife (Trohimovich, T. pg. 3 & Marsh, pg.
448). Therefore, it is crucial to maintain adequate buffers around riparian zones and wetlands
if we are to protect the waters and the fish and wildlife that rely on them.
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9.1 Surface Hydrology Methods
Lakes, streams, creeks, and rivers were identified and highlighted blue on our 1:12000 scale
topographic map, referencing the USGS Four Lakes 7.5 Minute Series to resolve any
ambiguities. Wetlands were then identified on our 1:12000 scale US Department of the
Interior, Fish and Wildlife, Service National Wetlands Inventory map and colored blue,
referencing the 1:24000 copy of the same map to resolve any ambiguities. Once all the
wetlands were identified, the 1:12000 National Wetlands Inventory map was placed on a light
table which was overlayed by our 1:12000 topographic map. We then transferred all the
water features onto the topographic map and colored them in blue. Using a ruler, a 1/3”
buffer was plotted around all the water features and colored in red.
9.2 Surface Hydrology Findings
We found that most of the hydrologic features of the landscape are along the entire border of
the study area, with the largest bodies and stretches of water being Fish Lake, Queen Lucas
Lake, and Minnie Creek. There are also a large cluster of wetlands in the northeastern,
southeastern, and southwestern areas of the study area. Lastly, we found that the center of
the study area is relatively void of hydrology features, due to the topography of the
landscape.
10.0 Final Composite
The McHargian suitability analysis informed much of the work that went into creating the
Final Composite Map for the suitability analysis of Prosser Hill. The results of this analysis
indicated that all areas were unsuitable for development. This map was created based off of
the SAS Composite Map, which set the initial limitations for soil permeability, depth to
bedrock, depth to water table, and slope. Also included in the Final Composite Map were
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agricultural capability and hydrologic information that was included from two additional
maps. These were all merged into one composite map combining all the severe, moderate,
and slight limitations. Again, each map was created based off of the severity of limitations for
the specified map classification which were: slight limitations, which was left as a white
field, moderate limitations, which was indicated by yellow, and severe limitations, which
were highlighted in red. The depth to bedrock map left no areas with slight limitations, so the
final composite map only includes severe limitations.
Another map was created based on the hydrological locations in the Prosser Hill area. 300
foot buffers were created around all streams, lakes, natural water bodies, and all wetlands. All
of these buffer areas are considered severe limitations for the suitability of septic systems and
were marked on the hydrology map in red. These buffers of severe limitations were also
added to the final composite map indicating more areas in red where the installation of any
septic tank systems was not suitable.
The color was transferred from the SAS Composite map, agricultural capability, and
hydrologic information maps starting with the most severe limitations to form new polygons
on the Final Composite Map. After transferring all of the red color for severe, the entirety of
the Final Composite Map was red. This final map composition leaves no developable acreage
in the entire Prosser Hill region.
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11.0 Conclusion
After a thorough examination of the Prosser Hill’s agricultural capabilities, hydrologic
features and constraints, and the soil absorption suitability of the area, it is our professional
recommendation to not develop in this area. Although some of the individual maps showed
what appeared to be moderate or slight limitations to development (e.g. depth to water table
map) once factored in with the remaining maps, it is clear that all of the land included in the
Prosser Hill area presents severe constraints to development. We recommend that the final
composite map be used for making the determination for development, because it represents
all of the data in our analysis, including: permeability, depth to bedrock, depth to water table,
slope, agricultural capabilities, and limitations due to hydrologic features. The combination of
all these elements, as displayed in the final composite map lead us to the conclusion that
development is not advisable.
12.0 References
1. Marsh, William M. Landscape Planning: Environmental Applications. Fifth ed. John Wiley & Sons, Inc., 2010, Print.
2. Design Manual Onsite Wastewater Treatment and Disposal Systems. Washington, D.C.: United State Environmental Protection Agency, Office of Water Program Operations, Office of Research and Development Municipal Environmental Research Laboratory, 1980. Print.
3. Soil Survey of Spokane County, Washington. Washington D.C.: USDA Soil Conservation Service & Washington Agricultural Experiment Station, 1968. Print.
4. The Land Capability Classification. USDA – SCS. Print.
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5. Trohimovich, T. Riparian and Wetland Buffers are not a “Taskings” Risk.
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