EXHBIT 10.4

                                PLEDGE AGREEMENT


          This  Agreement is made as of February 29, 1996 by EMCON, a California
corporation ("Debtor"), in favor of THE BANK OF CALIFORNIA, N.A. ("Bank").

                                    Recitals

          Debtor  and Bank  have  executed  a  Credit  Agreement  of even  dated
herewith  (as the same may be amended  or  supplemented  from time to time,  the
"Credit Agreement"),  pursuant to which Bank has agreed to extend certain credit
facilities to Borrower on the condition,  among others,  that Borrower pledge to
Bank and grant to Bank a continuing security interest in certain shares of stock
and other  property  as security  for  Borrower's  obligations  under the Credit
Agreement.  All capitalized  terms used in this Agreement that are not otherwise
defined herein shall have the meanings ascribed thereto in the Credit Agreement.


          1.  Grant  of  Security  Interest.  Debtor  hereby  grants  to  Bank a
continuing  security  interest in all issued and  outstanding  shares of capital
stock of each of Debtor's Subsidiaries  identified on Schedule 1 annexed to this
Agreement (as such Schedule may be amended or  supplemented  from time to time),
now owned or  hereafter  acquired by Debtor (the  "Pledged  Shares"),  all stock
rights,  rights  to  subscribe,  liquidating  dividends,  stock  dividends,  new
securities  or other  property  to which  Debtor is or may  become  entitled  to
receive  on  account  of  such  Pledged   Shares,   and  all  proceeds   thereof
("Collateral").  Except as  provided  in Section  5(d)(i),  in the event  Debtor
receives any of the above forms of property,  Debtor will promptly deliver it to
Bank  to be  held  by  Bank  hereunder  in the  same  manner  as the  Collateral
originally delivered hereunder.

          2.  Indebtedness.  Debtor  agrees that the  Collateral is and shall be
security for the timely  payment and  performance of all  obligations  under all
Indebtedness  to  Bank.   "Indebtedness"   means  all  debts,   obligations  and
liabilities  of Debtor to Bank  currently  existing  or now or  hereafter  made,
incurred or created,  whether  voluntary or involuntary  and however  arising or
evidenced,  whether  direct or acquired  by Bank by  assignment  or  succession,
whether due or not due,  absolute or  contingent,  liquidated  or  unliquidated,
determined  or  undetermined,  whether  under this  Agreement or  otherwise  and
whether Debtor may be liable  individually or jointly,  or whether recovery upon
such debt may be or become  barred by any statute of  limitations  or  otherwise
unenforceable;  and all renewals,  extensions and modifications thereof; and all
attorneys' fees and costs incurred by Bank in connection with the collection and
enforcement  thereof.  Any writing which evidences or is an agreement in respect


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to all or any portion of the  Indebtedness  is a "Loan  Document".  3.  Debtor's
Covenants. Debtor hereby represents, warrants and agrees that:

         (a) Debtor has acquired,  or forthwith  will acquire and maintain,  all
         portions of the marketable title to the Collateral described herein and
         will at all times keep the  Collateral  free of all Liens  except those
         permitted under Section 6.2 of the Credit Agreement;

         (b) Debtor will not sell,  transfer,  lease or otherwise dispose of any
         of the Collateral or any interest  therein to any individual or entity,
         including  without  limitation Bank where the context so permits and in
         Bank's sole discretion ("Person");

         (c) The Pledged  Shares have been duly and validly issued and are fully
         paid and  non-assessable.  Except as may be specifically stated to Bank
         in writing prior to the date hereof and as provided by law, the Pledged
         Shares are transferable without prior notice to, or approval or consent
         from, any Person or  governmental  or regulatory  authority,  and there
         exists no condition or  restriction to or affecting the transfer of the
         Pledged Shares;

          (d)  Debtor  will pay when due and  prior to  delinquency  all  taxes,
          levies,  assessments  or other  claims  which are or may become  liens
          against the Collateral;

          (e) Debtor will  neither  make nor permit any  material  change in the
          Collateral without the prior written consent of the Bank;

          (f) Except as otherwise  provided herein,  Debtor will deliver to Bank
          promptly  (i) all  Collateral,  (ii) all  proceeds of the  Collateral,
          (iii) such specific acknowledgments,  assignments, or other agreements
          or writings as Bank may request  relating to the Collateral,  and (iv)
          such  records  and other  reports  in such form and detail and at such
          times as Bank may require relating to the Collateral;  notwithstanding
          anything  to the  contrary  herein  contained,  Debtor may receive and
          retain  distributions  from a  Subsidiary  in  the  form  of  personal
          property  other than cash or securities in connection  with a business
          reorganization involving Borrower and such Subsidiary;

          (g)  Debtor  will  give  prompt  notice to Bank of any  threatened  or
          asserted dispute or claim with respect to the Collateral, any decrease
          in the value of any Collateral and the amount of such decrease  (other
          than  as  reflected  on  any  securities   exchange  or  other  market
          publication),   any   litigation  or   administrative   or  regulatory
          proceeding  which may have a material  adverse effect on Debtor or its


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          business,  and the  occurrence of any Event of Default or of any other
          development,  financial or otherwise, which might materially adversely
          affect the Collateral or Debtor's  ability to perform its  obligations
          to Bank; and

          (h) Debtor will  execute  and  deliver to Bank,  and file or record at
          Debtor's  expense,  all notices and other  documents from time to time
          requested by Bank to maintain a first perfected  security  interest in
          the   Collateral  in  favor  of  Bank,   all  in  form  and  substance
          satisfactory  to Bank,  and perform  such other acts,  and execute and
          deliver  to  Bank  such   additional   assignments,   agreements   and
          instruments,  as Bank may at any time request in  connection  with the
          administration  and  enforcement  of this  Agreement or Bank's rights,
          powers and remedies hereunder.

          4. Events of Default. The occurrence of any Event of Default under the
          Credit  Agreement  shall  constitute an "Event of Default"  under this
          Agreement.

         5.       Rights on Default.

         (a) Upon the occurrence of an Event of Default, all Indebtedness shall,
         at the option of Bank, without demand or notice, become immediately due
         and payable.  Bank shall have all other  rights and remedies  available
         under  contract or  applicable  law,  which  include those of a secured
         party under the Uniform  Commercial Code, at law, or in equity, and the
         right to take  possession  of the  Collateral  (if not  then in  Bank's
         possession),  and sell and dispose of the same, or any part thereof, at
         public or private sale.

          (b) The proceeds of any sale or disposition  shall be applied first to
          the  reasonable  expenses of retaking,  holding,  preparing  for sale,
          discharging  all liens,  selling and the like,  then to the attorneys'
          fees and legal expenses incurred by Bank, and then to the Indebtedness
          in such order as Bank may determine.  Notwithstanding the rights given
          to Debtor pursuant to California  Civil Code sections 1479 and 2822 or
          equivalent  provisions  in the  laws  of the  state  specified  in the
          governing  law  clause  of  this  document  (and  any   amendments  or
          successors thereto), to designate how payments will be applied, Debtor
          hereby  waives  such  rights and Bank shall have the right in its sole
          discretion  to determine  the order and method of the  application  of
          payments  received from Debtor or from the sale or  disposition of the
          Collateral   and  to  revise   such   application   prospectively   or
          retroactively at its discretion.

          (c)  Person(s)  liable for all or any  portion of  Indebtedness  shall
          remain liable for the unsatisfied  portion of such  Indebtedness,  and
          shall promptly pay the same to Bank  immediately  and without  demand,
          with  interest  thereon  at the rate  provided  in the  Loan  Document
          applicable thereto, or, if no rate is otherwise provided,  at the rate
          of interest  applicable to the unsatisfied  amount of a money judgment
          of a court of the state whose laws govern this  Agreement.  Should the
          net proceeds  resulting from any such sale or  disposition  exceed the
          amount  owing to Bank,  Bank shall pay such  surplus to the  Person(s)
          legally entitled thereto.

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          (d) So long as no Event of Default shall have occurred:

                           (i) Debtor  shall be  entitled  to receive and retain
                  all cash dividends  payable in connection with any the Pledged
                  Shares and to exercise any and all voting or consensual rights
                  and powers relating to or pertaining to any Pledged Shares for
                  any purpose not inconsistent with the terms of this Agreement.

                           (ii) Upon the occurrence  and during the  continuance
                  of an Event of  Default,  all  rights  of  Debtor  to  receive
                  payment  of  cash  dividends  or to  exercise  the  voting  or
                  consensual  rights and powers with respect to the  Collateral,
                  shall  cease,  and all such rights and  authority  to exercise
                  such voting or consensual  rights and powers or to receive and
                  retain such  dividends  shall inure to Bank. Any and all money
                  and other  property  paid over to or received by Bank pursuant
                  to the  provisions  of this Section may be retained by Bank as
                  additional  Collateral  or in Bank's  sole  discretion  may be
                  applied toward the satisfaction of the  Indebtedness.  In such
                  event, Bank shall have the right and power to receive, endorse
                  and  collect  all checks and other  orders for the  payment of
                  money made  payable to Debtor  representing  any  dividend  or
                  other distribution  payable or distributable in respect of any
                  Pledged Shares.

          (e) The  obligations of Debtor under this Agreement  shall continue to
          be effective or be reinstated,  as the case may be, if at any time any
          payment of any Indebtedness is rescinded or must otherwise be returned
          by Bank upon, on account of, or in connection  with,  the  insolvency,
          bankruptcy or  reorganization  of Debtor or  otherwise,  all as though
          such payment had not been made.

          6. Costs and Expenses.  Debtor  promises,  to the extent  permitted by
applicable  law, to reimburse Bank promptly for all costs and expenses  incurred
by Bank in  performing  any  agreement  of Debtor  which  Debtor  shall  fail to
perform,  or in taking  any other  action  which Bank  deems  necessary  for the
maintenance or  preservation  of any Collateral or its interest  therein,  which
costs and expenses shall constitute Indebtedness under this Agreement.

         7.       Power of Attorney.

         (a) Debtor hereby irrevocably appoints Bank, or any officer thereof, as
         Debtor's  true and lawful  attorney-in-fact  coupled  with an interest,
         with full power of  substitution,  to sign or endorse  any  instrument,
         document,  or other writing necessary or desirable to transfer title or
         other  rights  to or in  any  of the  Collateral;  and  to do all  acts
         necessary or incidental to assert, protect and enforce Bank's rights in
         the Collateral and under this Agreement. Debtor agrees that Debtor will
         reimburse  Bank  promptly  upon demand for any expenses  Bank may incur
         while  acting  as  Debtor's  attorney-in-fact,   which  expenses  shall
         constitute Indebtedness under this Agreement.

         (b)  Following  the  occurrence  of an  Event  of  Default  (except  as
         otherwise provided), without notice, and at the expense of Debtor, Bank
         in its name or in the name of Debtor may, but shall not be obligated to
         (i) collect by legal  proceedings  or otherwise,  endorse,  receive and
         receipt for all dividends,  interest, principal payments and other sums
         now or  hereafter  payable upon or on account of the  Collateral;  (ii)
         make any  compromise or  settlement  it deems  desirable or proper with
         reference to the  Collateral;  (iii) at any time,  insure,  process and
         preserve  the  Collateral;   (iv)  at  any  time,  participate  in  any
         recapitalization,   reclassification,   reorganization,  consolidation,
         redemption,  stock  split,  merger  or  liquidation  of any  issuer  of
         securities which constitute Collateral, and in connection therewith may
         deposit or surrender  control of the Collateral,  accept money or other
         property in  exchange  for the  Collateral,  and take such action as it
         deems proper in connection  therewith,  and any other money or property
         received  in  exchange  for the  Collateral  shall  be  applied  to the
         Indebtedness or held by Bank  thereafter as Collateral  pursuant to the


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         provisions  hereof;  (v) cause Collateral to be transferred to its name
         or to the name of its  nominee;  (vi) at any time,  exercise  as to the
         Collateral all the rights, powers and remedies of an owner necessary to
         exercise its rights, subject to Section 5(d).

          8.  Waivers of  Debtor.  Debtor  waives  any right to require  Bank to
proceed against any Person, or to exhaust any Collateral or to pursue any remedy
in Bank's  power  whatsoever.  Bank shall not be required  to make  presentment,
demand or protest,  or give any notices thereof,  or take any action to preserve
rights  against  prior  parties  with respect to any of the  Collateral.  Debtor
waives  the right to plead any  statute  of  limitations  or any  defense to the
personal  liability  of Debtor as a defense to Bank's  exercise  of any right or
remedy hereunder.

          9. Non-Waiver. Bank may, in the exercise of its sole discretion, waive
an Event of Default,  or cure an Event of Default at Debtor's expense.  Any such
waiver shall be subject to Section 11(c) below.

         10.      Bank's Duties.

         (a) Bank's sole duty with respect to the  Collateral in its  possession
         shall  be to use  reasonable  care  in  the  custody  and  preservation
         thereof.  Bank shall be deemed to have exercised reasonable care in the
         custody and  preservation  of such  Collateral  if such  Collateral  is
         accorded treatment  substantially  equal to that which Bank accords its
         own  property,  it  being  understood  that  Bank  shall  not  have any
         responsibility for ascertaining or taking action with respect to calls,
         conversions,  exchanges,  maturities, declining value, tenders or other
         matters  relative to any Collateral,  regardless of whether Bank has or
         is deemed to have  knowledge of such  matters;  or taking any necessary


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         steps to preserve  any rights  against  any Person with  respect to any
         Collateral.  Under no  circumstances  shall Bank be responsible for any
         injury or loss to the Collateral, or any part thereof, arising from any
         cause beyond the reasonable control of Bank.

          (b) Bank may at any time deliver the Collateral or any part thereof to
          Debtor  and the  receipt  of  Debtor  shall  be a  complete  and  full
          acquittance for the Collateral so delivered, and Bank shall thereafter
          be discharged from any liability or responsibility therefor.

          11. General Provisions.

          (a) Notices.  Any notices given or required under this Agreement shall
          be given in the manner specified in the Credit Agreement.

          (b) Binding  Effect.  This Agreement  shall be binding upon Debtor,its
          permitted successors,  representatives and assigns, and shall inure to
          the benefit of Bank and its successors,  representatives  and assigns;
          provided  however  that Debtor may not assign or  transfer's  Debtor's
          obligations under this Agreement without Bank's prior written consent.
          Bank  reserves the right to sell,  assign,  or transfer its rights and
          powers under this  Agreement,  in whole or in part  without  notice to
          Debtor.  In that  connection,  Bank may  disclose  all  documents  and
          information  which Bank now or  hereafter  may have  relating  to this
          Agreement,  Debtor  or  Debtor's  business,  provided  that  any  such
          assignee or transferee executes a confidentiality agreement reasonably
          satisfactory to Debtor.

          (c) No Waiver. Any waiver, consent or approval by Bank of any Event of
          Default or breach of any  provision,  condition  or  covenant  of this
          Agreement  or any  Loan  Document  must be in  writing  and  shall  be
          effective  only to the extent set forth in  writing.  No waiver of any
          breach or  default  shall be  deemed a waiver  of any later  breach or
          default of the same or any other provision of this Agreement or any of
          the  Loan  Documents.  No  failure  or  delay  on the  part of Bank in
          exercising any power,  right or privilege  under this Agreement or any
          Loan  Document  shall  operate as a waiver  thereof,  and no single or
          partial exercise of any such power,  right or privilege shall preclude
          any further  exercise  thereof,  or the exercise of any further power,
          right or privilege.

          (d) Rights  Cumulative.  All rights and remedies  existing  under this
          Agreement are cumulative to, and not exclusive of, any other rights or
          remedies available under contract or applicable law.

          (e) Unenforceable Provisions. Any provision of this Agreement which is
          prohibited or unenforceable in any jurisdiction shall be so only as to
          such  jurisdiction  and  only to the  extent  of such  prohibition  or
          unenforceability,  but all the remaining  provisions of this Agreement
          shall remain valid and enforceable.

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          (f)  Governing  Law/Waiver  of  Notice.  Except  as may  be  otherwise
          provided by the Uniform  Commercial  Code or in any  addendum  hereto,
          this Agreement  shall be governed by and construed in accordance  with
          the laws of the State of California.  To the fullest extent  permitted
          by law, Debtor hereby waives presentment,  demand,  protest, notice of
          dishonor and all other notices and demands,  as well as any applicable
          statute of limitations.

          (g)  Indemnification.   Debtor  shall  pay  and  protect,  defend  and
          indemnify   Bank   and   Bank's   employees,    officers,   directors,
          shareholders,  affiliates,  correspondents, agents and representatives
          (other than Bank,  collectively  "Agents") against,  and hold Bank and
          each such Agent  harmless  from,  all  claims,  actions,  proceedings,
          liabilities, damages, losses, and related expenses (including, without
          limitation,  attorneys' fees and costs) and other amounts  incurred by
          Bank and each such Agent,  arising  from the matters  contemplated  by
          this Agreement; provided, however, that this indemnification shall not
          apply to any of the foregoing  incurred solely as the result of Bank's
          or  any  Agent's  gross   negligence  or  willful   misconduct.   This
          indemnification  shall survive the payment and  satisfaction of all of
          Debtor's obligations and liabilities to Bank.

          (h)  Reimbursement.  Debtor  shall  reimburse  Bank for all  costs and
          expenses,  including without limitation reasonable attorneys' fees and
          disbursements  (and fees and disbursements of Bank's in-house counsel)
          expended or incurred by Bank in any arbitration,  mediation,  judicial
          reference,  legal  action  or  otherwise  in  connection  with (a) the
          negotiation, preparation, amendment, interpretation and enforcement of
          this  Agreement,  including  without  limitation  during any  workout,
          attempted  workout,  and/or in connection  with the rendering of legal
          advice  as to Bank's  rights,  remedies  and  obligations  under  this
          Agreement,  (b)  collecting  any sum which becomes due Bank under this
          Agreement, (c) any proceeding for declaratory relief, any counterclaim
          to any proceeding, or any appeal, or (d) the protection,  preservation
          or  enforcement  of any  rights  of  Bank.  For the  purposes  of this
          section,  attorneys'  fees shall  include,  without  limitation,  fees
          incurred in connection with the following:  (1) contempt  proceedings;
          (2)  discovery;  (3) any motion,  proceeding or other  activity of any
          kind in connection with a bankruptcy proceeding or case arising out of
          or relating to any petition  under Title 11 of the United States Code,
          as the same shall be in effect from time to time,  or any similar law;
          (4) garnishment,  levy, and debtor and third party  examinations;  and
          (5)  postjudgment  motions  and  proceedings  of any  kind,  including
          without  limitation  any  activity  taken to collect  or  enforce  any
          judgment.

          (i) Entire Agreement. This Agreement is intended by Debtor and Bank as
          the final  expression  of Debtor's  obligations  to Bank in connection
          with  the  Collateral  and  supersedes  all  prior  understandings  or
          agreements concerning the subject matter hereof. This Agreement may be
          amended  only by a writing  signed by Debtor and  accepted  by Bank in
          writing.

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          IN WITNESS WHEREOF,  Debtor has executed this Agreement as of the date
of the preamble.


                        EMCON

                        By:     /s/
                                ---------------------
                        Name:   R. Michael Momboisse

                        Title:   CFO & V.P. - Legal

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