PACIFIC RIM SOIL & WATER, INC.

Transcription

1 PACIFIC RIM SOIL & WATER, INC. Lisa Palazzi th Avenue East Olympia, WA (p) (f) December 16, 2008 Report Subject: Location: Preparer: Proposed Site Evaluation Protocol for LTA Lands Thurston County Lisa Palazzi, CPSS Introduction: Thurston County has developed an expansion of an earlier map (and regulatory classification) of what they define as Agricultural Lands of Long Term Commercial Significance (LTA as defined in Thurston County Comprehensive Plan, Chapter 3). Classification as LTA usually results in rezoning the subject parcel to a lower density typically converting from 1unit per 5-10 acres to 1 unit per acres. The earlier Phase I LTA mapping included only parcels greater than 20 acres that were in agriculture at that time. It is our understanding that this new Phase II mapping exercise includes other non-agricultural parcels that contain similar soils as in the original LTA map, and therefore could be successfully converted to commercial agriculture at some point. Detailed information about how the Phase I and Phase II maps were created is described in a March 19, 2008 Thurston County staff report titled: Response to Western Washington Growth Management Hearings Board Order to Designate Long-Term Agricultural Lands of Commercial Significance, Thurston County Planning Commission Public Hearing Draft. We did not review that mapping decision protocol in detail, so cannot comment on it. But in general, the maps were created first using Thurston County NRCS 1 soil map units in the GeoData coverage 2 that were classified as prime farmland (PFL 3 ) by NRCS. This new and updated Phase II LTA Thurston County GIS evaluation process identified the parcels that were mapped as having greater than 50% coverage of certain high quality Thurston County soil map units (list provided below) that had already been identified in the earlier process as having the required LTA characteristics. That map was further refined by removing some parcels having >50% coverage of slopes greater than 8% 4 (as per LIDAR slope analysis) and by removing some parcels having >50% coverage of mapped wetlands (in Thurston County GeoData coverage). Additionally, parcels less than 20 acres and contiguous with existing or 1 Natural Resources Conservation Service, formerly called Soil Conservation Service or SCS 2 GeoData is a hand-digitized version of the actual Thurston County Soil Survey. That soil survey is now available online at and should be consulted in concert the GeoData for best information. 3 Based on the NRCS Prime Farmland protocol 4 We provide, below, more details on this slope class selection criteria, and its effect on LTA assessment. Page 1

2 proposed areas of Long Term Agriculture were considered for designation. So the final map should include parcels most likely to meet the LTA standard, as defined by Thurston County in the document referenced above. Thurston County Soil Survey-based LTA Map Units NRCS Map Unit # Soil Series Name and Description 14 Bellingham silty clay loam (where drained)* 26 Chehalis silt loam 29 Dupont muck (where drained)* 31 Eld loam 36 Everson clay loam (where drained)* 37 Galvin silt loam, 0 to 5 percent slope 38 Giles silt loam, 0 to 3 percent slope 41 Godfrey silty clay loam (where drained)* 50 Kapowsin silt loam, 0 to 3 percent slope 64 Maytown silt loam 69 Mukilteo muck (where drained)* 70 Mukilteo muck (drained)* 71 Newberg fine sandy loam 72 Newberg loam 73 Nisqually loamy fine sand 0-3 percent slope** 75 Norma fine sandy loam (where drained)* 76 Norma silt loam (where drained)* 86 Prather silty clay loam, 3 to 8 percent slope 88 Puget silt loam (where drained)* 89 Puyallup silt loam 97 Salkum silty clay loam, 3 to 8 percent slope 100 Scamman silty clay loam, 0-5 percent slope (where drained)* 104 Semiahmoo muck (where drained)* 105 Shalcar muck (where drained)* 106 Shalcar Variant muck (where drained)* 107 Skipopa silt loam, 0-3 percent slope 115 Sultan silt loam 120 Tisch silt loam (where drained)* 126 Yelm fine sandy loam, 0 to 3 percent slope * Hydric or wetland soil (only prime if drained -- meaning that the wetland (hydric) soil has ditches or drain tiles that lower the water table during the growing season, so makes agriculture possible). ** Only prime where irrigated Under the Phase II rules, a parcel is regulated entirely as LTA if more than 50% of the parcel is classified as LTA. The only process that may be used to remove the parcel from LTA classification would be through onsite documentation (field work) that the portion of the parcel Page 2

3 that meets LTA standards has less than 20 inches of soil depth to a restrictive layer. Soil must be greater than 20 inches deep in order to meet the minimum depth standard for prime farmland. The NRCS specifically considers bedrock or glacial till to be restrictive to root growth, but any layer that causes water to perch above 20 inches for extended periods of time during the growing season enough to negatively impact common crops can also be considered restrictive in this LTA context. More information on the definition of growing season and long duration water tables will be provided below. We note that there are wetland soils listed in the table above, but they are only defined as prime farm land (i.e. included as LTA) if they are drained i.e., in this context, if their current growing season water table is > 20 inches below the soil surface. We should note that there are some crops that can grow with long-duration water tables above 20 inches, but these would not be considered common agricultural crops. So they would be ignored for the purposes of this classification. We also note that the Nisqually soil series is only included if currently irrigated. Since these new areas of the county defined as LTA are not supposed to be currently in agriculture, we may assume that they are not currently irrigated. However, that would have to be verified in the field. But, if the areas mapped as Nisqually soil series are not currently irrigated, according to information provided by James Weatherford at Thurston Conservation District, it would not be possible to obtain water rights for new irrigation. So those non-irrigated Nisqually soil areas would not be defined as prime farmland, and would be removed from LTA classification. Using the information above as a guide, we provide a suggested baseline protocol below that may be used in the field to determine if the area initially identified as LTA through the GIS process (described above) has enough soil depth (or in the case of Nisqually series is irrigated) under current conditions to meet that final requirement. We assume that this protocol will be used to potentially refute the LTA classification rather than to verify it since the request for field verification will come from the land owner. We assume that if the LTA classification is not challenged, it will stand. We should note that in some cases, there could be inclusions of other non-lta soils series in an LTA map unit that cannot be extracted without detailed and complex soil mapping, which is outside of the scope of this protocol, and possibly outside of the scope of the intent of the LTA program. Since this question may be asked at some point, we recommend that there be a clear decision as to whether that alternate approach (detailed soil mapping at the series level) may be accepted by the County. If so, only a CPSC, CPSS or RPSS (acronyms defined below) would be qualified to carry out that work. The generalized field work protocol that we offer below can be successfully carried out by the following properly trained professionals, ranked more or less in order of preference: A certified or registered professional soil scientist (SSSA 5 -CPSS, NSCSS 6 -RPSS) or 5 SSSA = Soil Science Society of America 6 NSCSS = National Society of Consulting Soil Scientists Page 3

4 certified professional soil classifier (SSSA-CPSC); A certified crop advisor with adequate soils training (SSSA- CCA); A WA state-licensed onsite septic system designer who has taken WOSSA 7 Continuing Education classes in soil profile interpretation with emphasis on restrictive layers and interpreting evidence of seasonal saturation; A certified professional wetland scientist (SWS 8 PWS) who has taken Continuing Education classes in hydric soil profile interpretation; A WA state-licensed Professional Engineer who has taken classes in soil profile interpretation with emphasis on restrictive layers and interpreting evidence of seasonal saturation; Because these are long-standing definitions related to agricultural soils, we use the NRCS definition of water table and long-duration and growing season (mesic 9 ) for this document. They are as follows: Water table: The upper surface of ground water where the water is at atmospheric pressure [i.e., the free-standing water surface in an unsealed bore hole or well at equilibration with atmospheric pressure]. the zone of saturation [i.e., all pores are full of water] at the highest average depth during the wettest season. It is at least six inches thick and persists in the soil for more than a few weeks. Long duration: a duration class in which inundation for a single event ranges from 7 days to 1 month. Growing season: the portion of the year when soil temperatures are above biologic zero at 50 cm (19.7"). The following growing season months are assumed for [the mesic] soil temperature regime of Soil Taxonomy: Mesic - March-October As mentioned above, this LTA definition process started with using the NRCS-defined map prime farmland map units. But Thurston County further refined that initial map be removing areas within those map units that were steeper than 8%. Slope affects erosion potential, and so affects a particular soil s ability to be farmed. That particular 8% slope limit is based on an NRCS water erosion potential rule used to qualify soil map units as prime farmland. It should be noted that there are several other site and soil characteristics also used to define prime farmland 1. water supply from precipitation or irrigation, 2. a favorable temperature and growing season, 3. an acceptable level of acidity or alkalinity, 4. an acceptable content of salt or sodium, 5. few or no rocks, 6. permeable to water and air, 7. not excessively eroded or saturated with water for long periods of time, and 8. does not flood frequently during the growing season or is protected from flooding, and other factors. So slope class alone does not define prime farmland. But using 8% slope as a screening tool is reasonable, since a soil s erosion potential 7 WOSSA = Washington Onsite Sewage Association 8 SWS = Society of Wetland Scientists 9 Mesic is Soil Taxonomy soil temperature regime terminology that describes our Puget Sound growing season, which relates to having dry summers and relatively mild wet winters. Page 4

5 generally increases with slope, making it less desirable as farm land. Based on NRCS guidelines, for a soil map unit to be classified as prime farmland, the soil map unit K factor 10 multiplied by the average map unit slope must be less than 2. For example, the Giles silt loam map unit 39 has a slope range of 3-15%. The average slope for the map unit is 9%; the K factor for that map unit is 0.32, and 0.32 x 9 = 2.88 (i.e. >2). So map unit 39 is not considered prime farmland. We provide a table below that defines the slope limit for the various listed Thurston County LTA soil series. Map Map Unit name (slope range) K Factor K x 3% K x 8% Unit # 14 Bellingham sicl (flat) Chehalis sil (flat) Dupont m (flat) 0.02 NA (only flat) NA (only flat) 31 Eld l (flat) Everson cl (flat) Galvin sil (0 to 5%) Giles sil (0 to 3%) Godfrey sicl (flat) Kapowsin sil (0 to 3%) Maytown sil (flat) Mukilteo m (flat) 0.02 NA (only flat) NA (only flat) 70 Mukilteo m (flat) 0.02 NA (only flat) NA (only flat) 71 Newberg fsl (flat) Newberg l (flat) Nisqually lfs (0 to 3%) * 75 Norma fsl (flat) Norma sil (flat) Prather silty clay loam (3 to 8%) Puget sil (flat) Puyallup sil (flat) Salkum sicl (3 to 8%) Scamman sicl (0-5%) Semiahmoo m (flat) 0.02 NA (only flat) NA (only flat) 105 Shalcar m (flat) 0.02 NA (only flat) NA (only flat) 106 Shalcar variant m (flat) 0.02 NA (only flat) NA (only flat) 107 Skipopa sil (0-3%) Sultan sil (flat) Tisch sil (flat) Yelm fsl (0 to 3%) The K factor calculation result using 3% (upper end of the prime farmland map unit slope class) compared to 8% indicates that the 8% slope screening limit is conservative. So use of the 8% slope as a GIS screening tool will still include all areas that could be broadly defined as prime farmland within a map unit. * Note that this information indicates that the steeper 3-15% Nisqually map unit 74 might be included as a prime farmland map unit; but that map unit does not qualify as prime farmland due to factors other than slope (as mentioned above). 10 K factor is in the Revised Universal Soil Loss Equation (RUSLE). Values of K range from 0.02 to Other factors being equal, the higher the value, the more susceptible the soil is to sheet and rill erosion by water. Page 5

6 Standard Field Protocol Tools and materials needed: Tile spade, hand-auger Munsell Color Book, field map with aerial photo background showing LTA portion of the parcel (less than 8% slope, mapped as one of the above soil series and not already mapped as wetland). On most sites, the following process should yield enough information to verify or refute the LTA classification. Step 1: Take GIS map to the field, and roughly verify if the boundary of the LTA portion as mapped is correct. If more than half of the parcel is >8% slope or is jurisdictional wetland or a combination of these two factors causes less than half of the parcel to meet LTA standards for depth and/or slope -- then the parcel should not be in LTA. Please note that on sites that are close to the 50% cut-off, survey-level information may be required to refute the LTA classification based on slope and presence of wetlands alone. In that case, the surveyor, working together with the field professional should provide a surveyed line showing exactly where the edge of the slope break and/or wetland boundary occurs on the landscape. If more than half of the parcel is upland and < 8% slope, then proceed to Step 2 to check on soil depth (or in the case of the Nisqually series, current irrigation) conditions on only the LTA portion of the parcel. Step 2: Hand-auger or tile spade soils to 30 inches depth across the LTA area at foot intervals. For a 20-acre site entirely within an LTA, that would result in around sample points. There should be no fewer than 8 sample points on a 20-acre site regardless. If no bedrock or glacial till (or other material that is impossible to hand auger or dig through) is encountered within 20 inches of the soil surface, then proceed with evaluation of soil color for indication of presence of shallow groundwater tables. Step 3: With a tile spade, cut an intact slab of soil profile from the side of the auger hole, and lay the slab flat on the ground beside the hole. Most tile spades are about 14 inches long, which will require a second deeper slab to see the entire profile to at least 20 inches depth. With flat palms, push sideways in opposite directions on the slab face to break the slab vertically to reveal undisturbed soil structure and colors within the slab. For the mineral 11 soil series (loams, silt loams, clay loams, silty clay loams in the list above): o If this field work is being carried out between March 1 and April 1, during a 11 Mineral soil versus Organic soil: A mineral soil is dominated by sand silt and clay; an organic soil has an unusually high content of organic materials from a slower than usual breakdown of plant materials. This second condition only occurs in wetland soils that are saturated for extended periods of time. The organic soil series map units are called mucks in the list above; the rest are mineral soils. Page 6

7 normal rainfall period and year, observation of a current water table at <20 inches below the soil surface across the LTA map unit area is enough to remove the parcel from LTA classification. o If the soil to at least 20 inches depth has a dominant Munsell Color Book chroma of greater than 2 with no rusty (reddish or orange) spots of color, then the soil meets criteria to be classified as LTA since there is no indication of a longduration water table or other restrictive layers within 20 inches of the surface. o If there are rusty (reddish or orange) spots of color at less than 20 inches depth below the soil surface, interspersed between a background Munsell Color Book chroma of greater than 2, then the soil is still expected to be well drained enough during the growing season to meet criteria to be classified as LTA. But it may have a periodic short duration winter water table events in response to winter storms. If in doubt, proceed to Step 4 to better define depth and timing of the seasonal water table. o If any of the soil layers within 20 inches depth below the soil surface are dominantly grey with a Munsell Color Book chroma of 2 or less interspersed with rusty (reddish or orange) spots of color, then the seasonal water table may be too shallow for the soil to be classified as LTA. These color combinations indicate long-duration saturation within 20 inches of the soil surface. In this case, proceed to Step 4. Please note that on sites that are close to the 50% cut-off in terms of soil depth similar to the slope class and jurisdictional wetland problems described above survey-level information may be required to refute the LTA classification based on soil depth alone. In that case, the surveyor, working together with the field professional will provide a surveyed line showing exactly where the edge of the soil depth break occurs on the landscape. For the organic soil series ( mucks in the list above): o Proceed to Step 4 if there is no current water table within 20 inches of the soil surface, since these soils are either currently wetlands or have been drained, and the current water table depth and duration during the growing season will need to be verified. Step 4: The color-based evidence of a shallow seasonal water table described above for mineral soils will persist for many years after a mineral soil has been effectively drained. And some of the organic soils, which were wetland soils, have also been effectively drained. So this final step is intended to document whether or not the hydrologic regime (long-duration water table) that created those soil colors or that soil organic matter content still persists above 20 inches depth. If it does, then the area should not be regulated as an LTA since there are severe restrictions to commercial agriculture with common crops. First, locate, sketch map and describe as precisely as possible any artificial (humanmade) or natural (nature-made) drainage features that may have lowered the water table indicated by the soil colors or organic content, as described above Page 7

8 o Artificial drainage: surface ditches or evidence of drain tiles (include depth and width of ditch or drain tile as well as water surface depth, if present), how the ditches or drains connect, the direction of flow, and where they enter and exit the target parcel etc. o Natural drainage: such as a deeply incised natural stream channel that may have drained what was once a wetland on a stream-side terrace. This documentation provides evidence that a previous hydrologic condition may not persist under current site conditions. If ditches are at least one-foot deeper than 20 inches, and they are well-maintained, that alone may be enough to indicate that the land is properly classified as LTA under current drainage conditions. Conversely, if the ditches are shallower than 20 inches, then it is likely that there is still a seasonal water table within 20 inches, so the parcel should not be classified as LTA. If in doubt, proceed to water table monitoring, as described below. If this work is carried out during the wet season, direct observation and measurement of the water table may be sufficient during mid-to-late winter months. o For work carried out during late winter months, direct observation and documentation that the water table is at greater than 20 inches depth over a period of at least 4 weeks prior to March 1 is an excellent indication that the site hydrology has been lowered to below 20 inches by the artificial or natural drainage features. In that case, the soils have been successfully drained, so the parcel DOES meet criteria to be classified as LTA. o If the water table is within 20 inches of the soil surface for > 4 consecutive weeks (as per the long duration definition provided above) during any period between March 1 and October 1 (mesic growing season, as defined above) in most years (at least 5 out of 10), then the area should not be classified as LTA since the area is soil depth limited. o To be classified as LTA, the water table may persist for long periods of time between October 1and February1 (not during the growing season), as long as it drains to below 20 inches by March 1 in most years. o On some sites, this documentation may require water table monitoring, either by regular manual measurements during critical periods of the growing season or through installation of programmable dataloggers that can record periodic water table levels (details provided below). If this monitoring indicates clearly that water tables persist within 20 inches of the surface for 4 weeks at a time after March 1 in most years, then that parcel should not be regulated as LTA. Other Issues: As mentioned previously, we note that several of the soil series listed as being potential LTA map units are hydric (wetland) soils. We have labeled those hydric soils for your convenience in the table. To meet this LTA standard, those hydric soils would have to be artificially drained to Page 8

9 the extent that their agriculture growing season (March 1through October 1) water table is below 20 inches. Under normal conditions, if these areas were in fact regulated wetlands, their long duration water table would be at 12 inches or less and would persist at least days into the early agricultural growing season in most years (i.e. would persist at least into mid-to-late March). In these soil types, proper interpretation of mottling patterns, and in particular, differentiating between water-related mottles and other mottles, can be a problem. But we assume that most of the people carrying out this work (as described above) will have the proper training to differentiate between a soil that has a current water table versus shows evidence of a past hydrologic condition or that they will know when they need to monitor to prove that condition. It is clear that the intent of this LTA classification is to include soils that have characteristics that lend themselves well to growing common agricultural crops in Thurston County. For future iterations of this process, we can provide additional comments and discussion as to soil seriesrelated data about potential crops. We have not provided that discussion in this document. WATER TABLE MONITORING Monitoring ports should be installed in the lowest elevation portions of the LTA portion of the site to document the worst case hydrologic condition. We note that the ports should be placed outside of wetlands, unless the wetland hydrology condition itself is to be documented. The intent is to determine depth to the water table in portions of the parcel that still may meet the LTA standard. In many cases, that groundwater table is flat rather than sloped, so this condition can sometimes be used to extrapolate the water table elevation horizontally to other nearby surfaces. Ports should be installed properly, but adapted to the 20-inch LTA standard, as per guidance provided in the Technical Standard for Water-Table Monitoring of Potential Wetland Sites, June 2005, by U.S. Army Corps of Engineers The water level in the ports can be hand-measured periodically. But in many cases, such a database is inadequate to properly document and assess long duration soil hydrology and response to storms. Recent improvements in technology make it possible and relatively affordable to use programmable dataloggers. A datalogger in at least one of the monitoring ports eliminates the great majority of the problems associated with the sporadic and low repetition sampling protocols of the past. Results from the datalogger can be plotted against local rainfall databases. Thurston County maintains several rainfall monitoring stations around the County. Graphed results provide a precise representation of the response and duration of spike events in response to real storms. They also provide a cross reference check for the other hand-monitored ports. As mentioned above, if this monitoring indicates clearly that water tables persist at <20 inches depth for 4 weeks at a time after March 1 in most years, then that parcel should not be regulated as LTA. Page 9

10 Conclusion: Some of this LTA site assessment work can be carried out during a relatively inexpensive onsite reconnaissance process, with costs of less than $1000. The areas that are properly identified as LTA are most likely to require the least work, and so the least expensive process. Areas where the LTA classification may be successfully reversed are more likely to be more expensive, since that documentation will take greater effort and possibly have to be carried out over a longer period of time. Costs could easily range into thousands of dollars in that case. It is not possible to provide any more detailed estimate since costs will depend on parcel size and project complexity. We hope this information is helpful. It is intentionally brief with few technical details. However, if needed, those technical details can be provided at another time. Respectfully, Lisa Palazzi, CPSS, PWS Pacific Rim Soil & Water, Inc. Page 10