TETRA TECH, INC. THIRD AMENDMENT TO NOTE PURCHASE AGREEMENT

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EXHIBIT 10.6

TETRA TECH, INC.

THIRD AMENDMENT TO
NOTE PURCHASE AGREEMENT

$110,000,000
Senior Secured Notes

$92,000,000
7.28% Senior Secured Notes,
Series A, due May 30, 2011

$18,000,000
7.08% Senior Secured Notes,
Series B, due May 30, 2008

Dated as of December 14, 2004

To the Holders of the Senior Notes
    of Tetra Tech, Inc. Named in
    the Attached Schedule I

Ladies and Gentlemen:

        Reference is made to the Note Purchase Agreement dated as of May 15, 2001, as amended by the First Amendment to Note Purchase Agreement dated as of September 30, 2001 and the Second Amendment to Note Purchase Agreement dated as of April 22, 2003 (the "Note Agreement"), between Tetra Tech, Inc., a Delaware corporation (the "Company"), and you pursuant to which the Company issued $92,000,000 aggregate principal amount of its 7.28% Senior Secured Notes, Series A, due May 30, 2011 (the "Series A Notes") and $18,000,000 aggregate principal amount of its 7.08% Senior Secured Notes, Series B, due May 30, 2008 (the "Series B Notes" and, together with the Series A Notes, the "Notes"). You are referred to herein individually as a "Holder" and collectively as the "Holders". Capitalized terms used and not otherwise defined herein shall have the meanings ascribed to them in the Note Agreement, as amended hereby.

        The Company has advised you that it will not be in compliance with Section 10.1(a) (Ratio of Consolidated Indebtedness to EBITDA) or Section 10.2 (Fixed Charge Coverage) of the

Note Agreement for the fiscal quarter ending October 3, 2004 or in the foreseeable future. The Company has requested an amendment of those terms and you have agreed to such amendment on the terms and and subject to the conditions set forth herein.

        In consideration of the premises and for good and valuable consideration, the receipt and sufficiency of which are acknowledged, the Company and the Holders agree as follows:

1.     AMENDMENT OF NOTE AGREEMENT

        1.1.     Amendment of Section 10.1. Section 10.1 of the Note Agreement is amended to read in its entirety as follows:

         "10.1    Adjusted Leverage Ratio; Priority Debt.

        The Company will not permit at any time:

        (a)    the Adjusted Leverage Ratio, as of the last day of any Computation Period, to be greater than: (i) 3.25 to 1 for any Computation Period ending before July 3, 2005; 3.00 to 1 for the Computation Period ending on July 3, 2005; or 2.25 to 1 for any Computation Period ending on or after October 2, 2005; or

        (b)    Priority Debt to exceed 15% of Consolidated Adjusted Net Worth."

        1.2.     Amendment of Section 10.2. Section 10.2 of the Note Agreement is amended to read in its entirety as follows:

         "10.2    Fixed Charge Coverage Ratio; Minimum Adjusted EBITDA.

        (a)    The Company will not permit the ratio, for any Computation Period ending on or after October 2, 2005, of (i) Adjusted EBITDA less Capital Expenditures to (ii) Interest Expense plus all income taxes (including any franchise or other tax based upon gross or net income or receipts) paid by the Company and its Subsidiaries plus all required payments of principal of Indebtedness of the Company and its Subsidiaries, in each case during such Computation Period (and determined on a consolidated basis), to be less than 1.25 to 1.

        (b)    The Company, will not permit Adjusted EBITDA to be less than the amount set forth below for any Computation Period ending on the relevant date set forth below:

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        1.3.     Amendment of Section 10.8. Section 10.8 of the Note Agreement is amended by deleting the word "and" at the end of Section 10.8(c), deleting the period at the end of Section 10.8(d) and inserting in substitution therefor "; and" and adding a new Section 10.8(e), to read in its entirety as follows:

        (e)    in no event may a Subsdiary be designated Unrestricted unless it is an Excluded Subsidiary, as such term is defined in the Credit Agreement.

        1.4.     Section 22. Section 22 of the Note Agreement (including Sections 22.1 and 22.2) is amended to read in its entirety as follows:

         "22. RESERVED."

        1.5.     Schedule B. The following definitions in Schedule B to the Note Agreement are amended to read in their entirety or are added, in appropriate alphabetical order, as follows:

         "Adjusted Consolidated Net Income" means, for any period, the net income or loss of the Company and its Restricted Subsidiaries for such period, excluding (a) extraordinary nonrecurring gains or losses and (b) non-cash impairment of property, plant and equipment and of intangible assets.

         "Adjusted EBITDA" means, for any Computation Period, the sum of Adjusted Consolidated Net Income for such period, plus, to the extent deducted in determining such Adjusted Consolidated Net Income, (x) federal, state, local and foreign income, value added and similar taxes, (y) Interest Expense, and (z) depreciation and amortization expense; provided that Adjusted EBITDA shall be calculated on a pro forma basis (in accordance with Article 11 of Regulation S-X of the Securities and Exchange Commission) giving effect to (a) any acquisition made by the Company or any Restricted Subsidiary during such Computation Period so long as, and to the extent that, (i) the Company delivers to each holder of Notes a summary in reasonable detail of the assumptions underlying, and the calculations made, in computing Adjusted EBITDA on a pro forma basis and (ii) the Required Holders do not object to such assumptions and/or calculations within 10 Business Days after receipt thereof; and (b) any divestiture of a Restricted Subsidiary, division or other operating unit made during such Computation Period. If the Company or any Restricted Subsidiary makes any acquisition of a Person or assets which would result in a negative adjustment to Adjusted EBITDA for any period, the Company shall, upon request of the Required Holders, deliver the information required pursuant to clause (a)(i) of the preceding sentence so that the calculation of Adjusted EBITDA will give effect to such acquisition.

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         "Adjusted Leverage Ratio" means, as of any date, the ratio of (a) Funded Debt on such date to (b) Adjusted EBITDA for the Computation Period most recently ended on or prior to such date.

         "Capital Expenditures" means all expenditures which, in accordance with GAAP, would be required to be capitalized and shown on the consolidated balance sheet of the Company, but excluding expenditures made in connection with the replacement, substitution or restoration of assets to the extent financed (i) from insurance proceeds (or other similar recoveries) paid on account of the loss of or damage to the assets being replaced or restored or (ii) with awards of compensation arising from the taking by eminent domain or condemnation of the assets being replaced.

         "Computation Period" means any period of four consecutive Fiscal Quarters ending on the last day of a Fiscal Quarter.

         "Consolidated Net Income" means, with respect to the Company and its Restricted Subsidiaries for any period, the consolidated net income (or loss) of the Company and its Restricted Subsidiaries for such period.

         "Fiscal Quarter" means a fiscal quarter of a Fiscal Year.

         "Fiscal Year" means any period of 12 consecutive calendar months ending on Sunday which is closest to the last day of September. References to a Fiscal Year with a number corresponding to any calendar year (e.g. "Fiscal Year 2004") refer to the Fiscal Year ending on the Sunday which is closest to the last day of September occurring during such calendar year. For example, Fiscal Year 2004 shall refer to the Fiscal Year ending October 3, 2004. Fiscal Year 2005 shall refer to the Fiscal Year ending October 2, 2005, and Fiscal Year 2006 shall refer to the Fiscal Year ending October 1, 2006.

         "Funded Debt" means all Indebtedness of the Company and its Restricted Subsidiaries, excluding (i) contingent obligations in respect of Guaranties (except, in each case, to the extent constituting Guaranties in respect of Indebtedness of a Person other than the Company or any Restricted Subsidiary), (ii) Hedging Obligations and (iii) Indebtedness of the Company to Restricted Subsidiaries and Indebtedness of Restricted Subsidiaries to the Company or to other Restricted Subsidiaries.

         "Hedging Agreement" means any interest rate swap, interest rate cap, interest rate collar or similar agreement protecting against fluctuations in interest rates, any currency swap, forward currency exchange agreement or similar agreement protecting against fluctuations in currency exchange rates, any commodity swap or similar agreement protecting against fluctuations in commodity prices, any option to enter i