8.0  Model Description    8.3.1  Tacoma Supply System    8.3.2  Reservoir Operations    8.3.4  Instream Flow Requirements   

8.3.5  Demand Forecasts    8.3.6  Conservation    8.3.7  TSI Intertie     8.3.8  Future Sources    8.3.9  Hydrologic Data

Tacoma can use groundwater to compliment surface water supplies.  When demand exceeds the allowable Green River surface water diversions, groundwater is extracted from over 20 In-town Wells to augment available surface water supply.  The In-town Wells (which include the South Tacoma aquifer wells, the Gravity Wells, and other smaller wells located within the service area) supply approximately 10-15 % of Tacoma’s annual water demands.  The combined maximum pumping capacity of these wells is approximately 60 mgd.

The largest groundwater supply is the South Tacoma aquifer, which has a maximum capacity of approximately 42,000 acre-feet.  CRYSTAL treats the aquifer as one underground reservoir (i.e., there is no distinction between the shallow aquifer and the deeper aquifer) with no shut-off for potential contamination.  Annual recharge of 12,000 acre-feet is distributed seasonally (Figure 8.19). Supply is limited by a maximum pumping capacity of 45 mgd, which is constrained by the time of year.  More water can be withdrawn during winter months than during the summer and early fall when drawdown and low recharge limit withdrawals.  The South Tacoma aquifer can be completely de-watered in CRYSTAL to demonstrate the effects of over-pumping.

In addition to the South Tacoma aquifer, CRYSTAL groups the other In-town Wells with the Gravity Wells.  These wells have a combined sustainable pumping rate of 14 mgd and include the Gravity Wells, Tideflat wells, Dash Point, University Place, and Portland Avenue.  Conjunctive management of the various groundwater sources is discussed below.

The North Fork wellfield, located 3 miles north of HAH, is used an as alternate source of potable water during winter floods when water in HAH can become highly turbid.  The volume generally ranges from 2-3 billion gallons and contributes approximately 10% of annual supply.  In CRYSTAL, the North Fork wellfield can be activated between early November and mid-February. When inflows into HAH are above a turbidity threshold (based on historical records from 1986-1993), a portion of the water is retained in the reservoir.  The North Fork wells are activated when water is retained above the flood rule curve to meet Tacoma's first water right.  In the summer when reservoir inflows are low, the North Fork wells are generally not used to meet the first water right because aquifer levels are also depressed.

Conjunctive Use of Surface Water and Groundwater Sources

CRYSTAL simulates conjunctive use between surface water and groundwater by assuming that Green River flows are used first to meet demands.  During periods of high demand and low flow, CRYSTAL assumes that Tacoma will begin to fulfill a percentage of demand by using in-town groundwater resources, even though there is no legal requirement that they do so.  This switch is currently triggered when storage for fish flows in HAH is below 8,000 acre-ft.  Under this scenario, Tacoma elects not to use inflows to meet a portion of their first water right, and this demand is met using groundwater.  In CRYSTAL, the Gravity Wells supply the minimum of either half their maximum pumping capacity or half of the demand deficit (i.e., demand minus Green River supplies). The South Tacoma well field is pumped only if demand is greater than the sum of the Gravity Well supply and the supply in the Green River.  However, if the South Tacoma aquifer is being stressed or if the current demand deficit exceeds 75% of the pumping capacity of the South Tacoma aquifer, the Gravity Wells are pumped at full capacity.

Because the North Fork well field is not used to augment the first water right during low flows, any deficit between demands and Green River diversion is made up through the Gravity Wells first, and then the South Tacoma wellfield.

When Pipeline 5 comes online, the same water use priority will hold: deficits between demand and the combined surface water supply from Pipeline 1 and Pipeline 5 will be complemented with groundwater.  In CRYSTAL modeling the expansion of HAH, as explained in the discussion in section 8.5.7, will also trigger an additional switch to In-town Wellfields.  This action will be triggered when stored water has been used to meet downstream baseflows and Tacoma "owes" this amount of water to the Corps so that the Corps can continue to meet its instream requirements at Palmer.

Table of Contents    8.1.1  Everett Supply    8.2.1  Seattle Supply System