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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